I've never seen or met anyone who had actual implants in her ass, but of course I know a ton of girls who have free silicone injected and I do not suggest going that route due to the various complications that can and will happen. I will tell anyone it's not worth it but of course most girls will just go for it anyway in order to get a more feminized body.
I have seen a good result from a fat transfer--where fat is harvested via liposuction and injected into the butt. This depends on whether or not she has enough fat to harvest though and also her ass may or may not retain the transferred fat. This is sometimes referred to as a "brazilian butt lift".
Of course, I always recommend going the natural route of exercise. Doing squats and lifts to target her ass can make a big difference.
Believe me, let her know that going to a board certified plastic surgeon in a sterile environment is the best way to go. Like I said I do not recommend injectable silicone at all--some girls I know seem to have no problems yet, and some have had moderate to severe problems (including me) with it. Of course we always hear on the news about deaths caused by illicit silicone quacks injecting girls with industrial-grade silicone they got at Home Depot.
I hope whatever she chooses she'll be happy with the result.