Having a stutter myself, I’d encourage you to speak up when you hear people make fun of a buddy. Different things work for different people, but I like to be very honest w/ people I confront. Something like, “Hey guys, over heard you talking about ______. I know you don’t mean any harm by it but as someone who stutters it hurts hearing you crack those jokes. We can’t control our stutter and it sucks really bad. Do you have any questions about stuttering? Not many folks know a lot and I’m happy to talk about it.” My purpose for saying something like the above let’s them know 1) it isn’t cool to continue doing that, 2) it hurts and something that it’s just shaken off, and 3) attempts to educated them. The dudes genuinely probably don’t know a thing about the disorder. I’d bet after being confronted and letting them know you’re human, they’d thing twice about how they crack jokes in the future. Just my 2c. : )