Videre non videri
Join Date: Sep 2004
Location: Gothenburg, Sweden
Bikes: 1 road bike (simple, light), 1 TT bike (could be more aero, could be lighter), 1 all-weather commuter and winter bike, 1 Monark 828E ergometer indoor bike
Haven't used a trainer myself, yet, but from what I've read, nothing but smooth tyres will work well, so you may end up having to get a second rear wheel, mount a slick tyre on it and put that on her bike every time.
However, one other option is to buy any cheap old road bike, give it a nice new rear tyre and have her use that. I assume you're too disparate in height for her to be able to use your bike - because with one trainer, only one person at a time will be able to use the trainer any way...