Should I kindly ask people to shut up when they make fun of my stuttering friend?

Should I kindly ask people to shut up when they make fun of my stuttering friend? Yesterday I went to get food with a very good friend of mine. He had had an exhausting and difficult day and stuttered more than usual. So when he went to order, it took him a little while to finish his sentence, naturally. The food guy immidiately started making fun of him ("We don't serve p-p-p-p") and actually said some really nasty stuff ("Did you fall on your head, boy?"). It made me really angry because I know how much this kind of asshole behavior hurts my friend... but I didn't say anything because I think that maybe he wants to deal with this himself? I've seen him immidiately talk to people who were being dicks, telling them that he stutters and that it would be nice if they could just wait until he's finished. But that day, he didn't say anything until the guy did it for the 4th time in 15 minutes - and even then he was still being condescending instead of apologizing, making it seem like he _mercifully_ tolerates the stuttering. So I wonder: Should I have stepped in earlier and "protected" my friend? I don't usually see him as someone who needs protection, actually the other way around, but this time it was different somehow, it didn't feel right just watching this guy be an asshole towards one of the kindest people I know. How would you want a good friend to react? I'm asking here because I'm not sure how comfortable he is with me asking about his stuttering all the time.