Ever heard of Squaters Ass?
Combine squats with food and her ass WILL grow. No reason to complicate things, though obviously a few other lower body exercises are good but squats should be the focus.
I come from a family of flat assers… I’m all leg, but grew a round firm bum with a ton of weighted walking lunges - as much ROM as possible in good form, uphill are even better, sprints on stairs as HIIT cardio. After having built a bum I moved onto 1 leg squats (pistols) which made my glutes REALLY sore.