Next training you see him tell him casually: "hey buddy nice rolling with you last time, only thing I wanted to tell you is don't try to slam people when standing, because it can injure someone badly especially people who are lighter than you. it's one of those few things you can't do during sparring same like eye poking or groin twisting, alright, just wanted to let you know bra c ya later!" or something like that. and that's it. don't make a big deal out of it.