Girlfriend's friend gets touchy

So my girlfriend has this one friend who has always been touchy feely, since before I even came into the picture. When we first started dating she warned me about it. He has this obsession with her ass, to be fair she has a really big butt for a girl her size (very athletic). We live together now and every time he comes over he'll slap or grab her ass at least a few times. But, is it normal I don't mind? I've even noticed she tends to wear short shorts or yoga pants when he's there or she knows he's coming over. I trust there's nothing going on and my friends dont understand but I think it's no big deal.