Get actual butt implants from a board certified plastic surgeon.
The people who do silicone implants are usually shady unlicensed non-professionals who set up "implant parties" and do a whole bunch at once. It's extremely unsafe and you can end up with a deformed looking ass or an infection that lands you in the hospital.
Your friend could also get those butt pads and fake a nice round booty. Of course, that only works if the clothes stay on. Once you're naked, the jig is up.
