IMO, this is a bit like buying some shoes. If you know their size and tastes, great; if not, then it's a big risk. Me, I bought a used bike to see if I wanted to get into bike; the fact that it might not be the right size or has ancient components didn't bother me. My wife, OTOH, stated up front that if it didn't fit her, she just wasn't going to ride it much. [I guess she doesn't like discomfort--this from a women who has had two kids?]

This is a hard call; without trying one out, you'll never know if it will work. Buying used really limits the choices (and some people tend to take offense on given "used" gifts); yet you have indicated that new LBS is probably out of your price range. I'm afraid that she will have to try the bikes out period, unless if you find a screaming deal on a size that you think will fit her (and the gift comes with a card that promises a better bike if this one doesn't work out).