stair climber or stair steppers at the gym, 40 minutes a day (4 days a week) for a year will do the trick. Many of my female friends with juicy booties swear by the stair steppers, and they feel nice too
I don't know much about injections but from what i've seen sometimes asses look deformed possibly from the constant sitting on it. Kind of look like bad boob jobs, kind of tumorous. But obviously there are good butt jobs out there.
It's a shame she is thinking of doing it with silicone. Nothing wrong with small butts. Try hitting the gym and getting some muscle on that booty it will be cheaper and last longer and feel better. Good luck