When I was bike shopping recently, I tried the Trek hybrids. I'm 5'7. The regular 20 inch (not WSD) fit me well - I have long arms and a long torso and felt a little hunched on the WSD. I'd imagine a 20 in. WSD would be a good fit for most regular gals. The 17.5" definitely felt too small.

For what that's worth!

But...if it's properly fitted, she wouldn't be able to touch the ground with both feet while on the seat. If the seat is that low, she'd end up with a lot of leg pain when actually riding. Or am I misreading you there?