Raccoons are tough. I used to hunt them back in my younger days. We would take a boat out into the bayou at night and spotlight them in the trees near the water. I can tell you from personal experience they can be pretty hard to kill even with a .22 or a shotgun. It's always a nice surprise when a big one you thought was dead suddenly stands up pissed off in the middle of a small boat between you and your buddy so you can't shoot it.

I have killed a couple of birds with a slingshot. I would be amazed if you could kill a raccoon with one.

Tell him to get one of those live traps and just catch them and haul them off.