Is it stuttering? The problem with stuttering is that feeling nervous about it makes it much worse. Ironically the best way to stop stuttering it to accept it; to not care whether you stutter or not. Family and friends can help best by not making a big deal about it. Be supportive, respectful and listen. The best thing for someone who stutters is to have friends and family who don't care about stuttering.
I stuttered pretty badly through my adolescence and early twenties. It made social situations awkward. The worst thing for me was that worrying about stuttering makes a little "catch" into a big stutter. I used to change words up or elide vowel sounds, all of those tricks made it a big deal and made it much worse.
In my early twenties I decided to stop giving a **** (I think I got this out of a book, but I don't remember the title).
I went and purposely stuttered in the very situations where it was most painful. I would find the most attractive female or intimidating male and I would stutter on purpose. It was my little game. That way instead of being of being scared, I controlled the situation. I knew what would happen, and I was doing it on purpose. And, it made it a joke where my stuttering wasn't the punch line.
That actually worked for me. Once I learned to stutter on purpose, I stopped caring about whether I stuttered or not. It became a non-issue.
These days, I still get caught on words once in a while, but now it is never more than a single syllable catching one time. And, once in a rare while someone has said something to me about it. But it is no longer painful and no longer any impediment to my social life.
That's just my experience. There are people who struggle with stuttering for all their lives. The most important thing is having friends and family who accept you for who you are (but this is true whether you stutter or not).
Some people have had good luck with therapy, I suspect that successful speech therapy will do the same desensitization process that I went through.
I think the research suggests that stuttering is largely genetic. The best thing you can do, as a friend, is to not give a ****.