I dont understand why you think you need a custom frame. Any xs or size possibly size s will work just fine.
There is no such thing as having correct leg extension and being able to touch the ground while in the saddle. You are supposed to hop off the front of the saddle and straddle the bike as you come to a stop.
If she doesn't want to do it, either ride without full leg extension or make due with your two feet and a pair of sneakers.