17" for a mountain bike for a 5'7 inch person seems fine or a little big. I'm 5'1 & ride a 13-14" mountain bike, while a friend is 5'6 & rides a 15/16" mountain bike - they have a 33" inseam though. So in general, I'd say a 17" frame should be fine for her, but there's variations given specific geometry of that frame and your gf's specific measurements, which we don't know.

Anyways. For mountain biking, you want a smaller frame with more standover that you can kind of throw or haul around easily. You can fit a road bike frame much bigger.

You could have her professionally fit to the bike. is she complaining about anything on long rides? You can change the length of the stem to make the top tube longer (or at least the handlebars further away). You can change the handlebars and how much rise or sweep they have. You could put a setback seatpost on it as well. But before you do all that, I think you should find someone who knows about fit.

If you post pics of what your gf riding the bike, we can give you more comments... but once she starts doing longer rides, I'm sure she'll give you feedback on what hurts