I'm a very extroverted stutterer too and back in high school I was in the band so I knew a good 150 people just from that. I played percussion and this other guy (lets call him S) was also in percussion. I was told by maybe 50 people that S would mock my stutter behind closed doors. ( I actually found out later but apparently the guy is one of those people that hate people with disabilities and thinks they shouldn't have kids. That, or the fact that I was always way better at percussion than him and always getting the parts he wanted, probably contributed to him not liking me). I would never have known if the 50 people haven't told me. And honestly, it never bothered me cus I have thick skin when it comes to stuttering. But whenever he would mock me, the people around him would defend me and stop being his friend. Most likely his friends have told him about the people that mock him, and he will probably go about his regular life. Sorry this comment is kinda long, but you get the just. You're friend probably knows and he's perfect with it. He's got real friends and he knows who they are.