If a pack fits and rides well, yes an extra lb *gasp* could be worth it.
Often in women specific packs the weight is in the hip belt, which is more padded and contoured than a similar man's version. The extra padding cuts down on hip owies down the trail - which come from rubbing.
If you go under 2 lbs you won't have good hip padding. As for weights....you can get many women specific packs in the 30L range for between 2 and 3 lbs. You just gotta really look, and no, you won't find them in the store.
At 5'1" with a 15" torso, that leaves your GF with a decent sized torso. Where is the packs tried on not fitting? (Reason why I say this, many women that short have smaller torsos, like a 14") Not all women's packs are even designed for under 15 1/2" torsos either. Many are 15 1/2" to 17". You need to look carefully on that, some are multiple sizes. As well, certain brands have smaller hipbelts (GoLite has very small ones) where as Dueter has larger ones but they also offer tinier torso fit.
You can still come under 3 lbs without issue.