I'm about your gf's size and I'm always keeping an eye out for frames my my size. Not that I need any more bikes (I have 7).
I haven't seen any lately that I can point you towards.

Nashbar makes a $99 aluminum mtn bike frame but I think they are out of the smallest sizes right now.

Check ebay. I have gotten a few nice frames my size there over the years. They usually go cheap as few people need a frame that small.

I have a couple mid-1990s Novara Randonee steel touring bike frames that I love. They used to make the xxs/xs sizes with 26" wheels back then. I have both built up as commuter bikes. I got one of the frames on ebay but they might show up again from time to time.