mtn bikes are a tough purchase if you don't already know what you want, or haven't owned one before. There are so many options that you can easily become paralyzed from over analysis (in my opinion)
In addition to previous replies - which I agree with - you also want to consider the type of riding terrain that it will be used for. Some people buy a mountain bike with the intent of it being an all-purpose bike. And some want to do extreme downhill with 6' drops. And there is a full spectrum in between.
My recommendation - and the bike I have - is a straightforward steel or alloy frame, hardtail (no rear suspension) and a front fork with suspension. Most ladies don't need a 29'er so don't even worry about that set of bikes. Tires can be swapped out if needed but probably unnecessary.
Speed - that's up to you mostly, or the grade of the descent
performance - depends on her riding level and experience. I like shimano XT equivalent, but I ride a lot.
price - again depends. If she's new to it, find the cheapest entry level bikes, and then go one performance level higher.
material - aluminum is usually your best bet, but this also depends
good luck.