As a 5'7" man with long legs and short torso I struggle with bike fit just like most woman do. I've managed to make all of my bikes fit with just a little modification to stem length. At 5'7" you should not have much problem fitting her properly to a regular CX bike as long you start off with the right size. I would look for bikes around 51-52cm, top tubes around 530, 535 at the very most. Most 49cm bikes will have a shorter head tube and with long legs excessive saddle to handlebar drop becomes an issue. A decent shop should be willing to change the bar to one that is narrower and change the stem to one that is shorter. A 400mm c-c handlebar and a 80-90mm stem.
I've fit woman to cross bikes with great success, you should be able to do the same. The 5'0"-5'3" woman is where it becomes really difficult.
Last edited by Saddle Up; 02-09-10 at 08:49 AM.