I’d skip the .12 gauge, but you might have her try a .20 if you can borrow one. I’m surprised how some girls go for larger calibers than you’d expect them to.
My daughter is 5'7" and barely weighs over 100lbs. She likes to use my old Western Field (Mossberg) 12 gauge pump that my dad bought for me in the early 70s. She can break a lot of clays with that thing. I tried to get her to use a 20 gauge but she did not like it much.