I chastise stutterers who curse themselves for stuttering just as I'd chastise someone for taunting a stutterer. If your friend beat himself up for getting stuck on a word I'd say 'Hey don't talk that way about a friend of mine' and be sure to let him know I won't tolerate talking himself down in my presence. Something you might consider. I've seen a lot of stutters just rip themselves apart with self loathing just for stuttering. It can be extremely toxic. If we can, we need to help them move away from that kind of self destructive talk.