Check her proportions. I *look* like an average sized woman at 5'6", but my legs are relatively short (29-30" inseam). I can use a small framed men's bike with either 700C or 26" tires. It's not unusual for a woman your girlfriend's size to have legs just as long as mine, but a shorter torso. That makes a "men's" (better described as a long torso) frame not as appropriate for her. The reach will be long enough that it's uncomfortable. There is a fair bit that can be done to help, but it costs. That's why a lot of smaller women tend to be very much in favor of WSD (aka short torso) bikes. If you have to swap bars, stem, brake levers, shifters etc several times to get comfortable, it can be very expensive.

(this isn't a gender thing, because there are a lot of men who have shorter torsos and longer legs too. since most WSD designs stop at too small, they never get to try it and see if it helps. very silly, and they can end up spending just as much getting a good fit)

Folding bikes can have similar problems. The big advantage is the small wheels mean the designer has options in getting the geometry right for someone who is on the small side. The big downside is a lot of folders are one size fits most, so the reach might still need modification.