I want to buy a Elite Suito T smart trainer that I would share with my girlfriend.
My bike is a Specialized Allez Elite 2016 - Shimano tiagra with 12-28 cassette.
My girlfriends bike is a Specialized Dolce Elite 2016 - Shimano tiagra with 11-32 cassette.
Now in order to be able to both use the smart trainer and swap one bike with another, I was told I would need to get a 11-28 cassette for the trainer but that her bike might not fit 100% on it. What do you guys think/recommend?

At least you both are running 10 speed cassettes. I think the issue would likely be that her chain is too long for the 12-28 cassette, so she'd want to be avoiding low gears on the small chainring while on the trainer. There's I guess a small possibility you could split the difference and put an 11-30 cassette or somesuch on the trainer and you get lucky, though not sure who makes them or if they exist.

Thanks for the reply,
Could I simply buy a 11-28 or 12-28 cassette for the trainer and also change her bike cassette for a 11/12-28? I would need to readjust her speeds I believe but would it simplify her experience?

You could, but you might also need to remove a link or so from her chain to fit the smaller cassette.
Do you both have the same cranksets in the front (eg. 34/50 tooth rings)?

Yeah we have the same in front. So yeah I guess her chain is too long

I'd say just try it first as you have it, but... carefully. On a trainer, you may well find you don't need every gear available, especially the extremely low 1-2 gears which I think might be the only issue.

SCRATCH that.. the problem is using the small front chainring with the smaller rear cogs that need to be avoided (which is a somewhat mid-range gear). And on a trainer those could be found useful but should be avoided.

Her chain isn't too long. If everything is 10sp, you should be fine. At most, you'd have to adjust her B Screw as you swap between trainer and rear wheel, but I doubt that even that's necessary.