If she has back and orthopedic issues, you will not be able to determine the proper fit and comfort without putting her on various bikes.
Don't be pennywise and pound foolish. Either sacrifice her comfort for a few bucks or go to a decent LBS. Hybrids aren't expensive enough to get much in the way of discounts unless you luck into just the right used bike. Even so, she has to hae the right fit which cannot be accomplished through the internet.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.