If she has to pick one, the Semi-Hollow is likely to give her the widest tonal range. The Semi-Hollow is a bit more like weight relief so keeps it on the lighter side. The Semi-Hollow offers 12 pickup selection options and can get closer to the more classic instruments (like a LP, Strat, tele) with the various options and the Pick-ups, pick up placement. I am NOT saying they can sound the same as having those instruments at all but its going to get you closer to those 'classic' sounds but has more of its own character coming through. The Humbuckers are tapped and you can't use the Narrowfield Humbucker on its own - without modding.
The Custom 24-08 (the 35th Anniversary is basically a 24-08 with a different knob/switch layout so I am using 24-08 to mean 'either') is more modern and more of its own thing than being a bit more Les Paul like. Yes you can split the Pickups individually too but the 24 frets mean that the neck in particular has a different sound than a lot of other double humbucker guitars. Of course there are several options that offer the 'same' 8 pick up selection choices - the Pauls guitar, the 594 and the 408 for example that all also have their own vibe going on too from the Vintage inspired 594 to the more modern 24-08. Really, these are all double humbucker guitars with their own vibe and feature set and picking comes down to what 'vibe and/or features you want - trem or hardtail, vintage, classic or modern?
There is also the 509 which I think is probably the more versatile in terms of being a single guitar instead of having to take a LP, Strat and Tele to a gig IF that's what you want. Not to say the SSH won't, I just think the 509 is closer to that and the SSH is more of its own thing. The 513 too is a fantastic instrument but was replaced by the 509 so the only option is to find a used one now. You have 3 'voicings' of the HB's from Single Coil, to Clear (lower output) to Heavy (higher output) so a very versatile guitar.
The most versatile without a doubt is the Modern Eagle V. Its got 17 different pickup selection choices from its 'HSH' configuration and each of those can be tweaked with a single switch between 250k or 500k volume pot so gives you 34 different choices. Of course its more expensive, but if you are looking for the '1' guitar for all (or at least the majority) of guitar sounds, this would probably get you in that ball park. I think this would get you closer to that Stratty position 4 for example than the 509 or SSH - especially with the 250k switch.It has the same '9' options as the 509, as well as 4 from being able to use both Humbuckers together (HH, HS, SH, SS - like Cu24-08, 594, SSH etc) and another 4 from having all pickups active (HSH, HSS, SSH, SSS) and, like I said, every one of those can be switched between the 250k (more usual for a Strat) or 500k (more usual for double HB guitars)
Of course, all of that depends on whether she is 'tone chasing' - ie looking for a single guitar that can get close to being like a Les Paul, like a Strat etc - or whether she is just looking for a guitar that offers a 'great' range of tonal options in one to offer her as much choice of usable sounds as possible from a single instrument. Every single one of these is a fantastic instrument in its own right and you cannot really go wrong with any.
I see a lot of people that expect a Split Humbucker (therefore essentially a Single Coil) to sound exactly like a Strat (or Tele) and then are disappointed when their shorter scale, predominantly Mahogany with set neck build and 500k volume pots doesn't sound exactly like a strat. I can understand why as SC's are associated predominately with Strats but they are a different build too. If that is what she is looking for, she maybe better off buying a Strat (Silver Sky) and picking up any of the double humbucker guitars, whether they have splitting/tapped options or not. The Hollowbody II could be a great option - just a 3 way switch for the Magnetics, but also a Piezo for that 'Acoustic' sound too and its very light weight. In my opinion, PRS are the best at coil splitting/tapping - not because that makes them sound like a strat, but because they don't lose a lot of volume and are very usable tones in their own right. I am always wary when people are looking for a 'single' instrument with as many options as possible. It makes me think that they want a guitar that can sound exactly like a Les Paul and Strat in one guitar rather than say a Custom 22 with more tonal options to be creative with. I know people say its more 'stratty' or more 'tele' like but I think of it as a sliding scale and its not '100%' Strat - it maybe closer to the strat sound when Split than in Humbucker mode.
Point I am trying to make is that having the option to split Humbuckers doesn't mean you will have a guitar that can be the Les Paul in full HB mode and a Strat in SC mode - but that doesn't mean that the SC mode can't be its own thing and be very useful in its own right. I don't know what you/she is looking for or needing. However, the SSH and Cu24-08 are incredible instruments and offer a lot of tonal variety. They are quite different instruments and have their own character so if she is looking more for something she can use for creating her own sounds, having 'more' options in one guitar rather than say chasing a particular sound, then either of these would do - but its almost like asking whether or not you should buy a Strat or Les Paul - maybe not quite as extreme as that, but they really are tonally quite different with a different vibe too.
At the end of the day though, she should really try them herself and decide which feels the best in her hands, which has the sounds she prefers and which will be the best fit for the music she creates. If she is just considering the Cu24-08 vs SSH, the neck and 'vibe' of each is very different and as such, she should pick the one that fits her the best (tones, comfort etc). I have a regular Cu24 (same Pups) and a SSH too and can easily justify owning both because the vibe is different and hey both have their own 'role'. I also have a 509 too and that's my most 'swiss army knife' guitar despite having fewer options than my SSH. The 509/513 have a longer scale length and great for covers bands who don't want (or can't) keep swapping between different guitars through a gig. The 594 is a shorter scale with a thicker neck but still offers '8' options like the Cu24-08 and Pauls guitar - its a beautiful Vintage inspired instrument without the issues, hassles and price tag of the instrument its inspired by and Paul's guitar is another double humbucker, 8 tonal option guitar with a different vibe (and neck carve) too. The MEv has the same neck and scale length as the SSH but the most versatile guitar I have seen or heard. I think its probably the 'best' option for anyone who wants the most tonal variations from a single guitar. The Special is a very special instrument but no longer in production so you may struggle to actually try one out in person.
Its her that has to play it and use it in studio so it really should come down to which guitar fits her the best. I know this is a long post that doesn't give you a definitive answer as to what guitar she should opt for - and maybe even added more options to make the decision a bit more complicated but I also hope that I have provided some useful information that may help too. I appreciate it isn't easy to get out to a retailer to try them in person right now and therefore I would suggest you try and find some videos, not just PRS made videos, of these guitars being played to try and at least gauge which may be has the vibe/tones you prefer - the more you watch, the better as it gives you a broader range of gear its being used with. Listening to one may sound really good/bad but that could be down to their own preferred set-up for example and very different to your experience if you buy it and run it through your own set-up.
