When he’s trying to talk and can’t get any sound out, that’s called a block. The best thing you can do is just wait for him to finish talking, like you would with anyone else. My husband stutters and has a lot of blocks. If you just listen to your friend and focus on what he’s saying, you’ll get used to how he talks pretty fast. I don’t know if your friend has social anxiety or not, but social anxiety doesn’t cause stuttering. Stuttering can cause people to be anxious in social situations, though. Your other friend means well, but saying things like “take your time” doesn’t help. I think a lot of well-meaning things people say come across to the person stuttering as condescending. I would say that it’s okay for you, maybe if you are talking to this guy one-in-one, to ask him if there’s anything he’d like you to do. Don’t put him on the spot about it or bring it up around other people, but it’s also okay for you to ask him about, as long as you are genuinely wanting to know how to be a good friend and won’t push it if he doesn’t want to talk about it.