PGA Professional Ted Eleftheriou, offers some suggestions:
Strive to maintain your composure
Don’t attempt to offer tips in a volatile situation
Do try to defuse the situation with positive words of encouragement
When all else fails, try a little humor to distract the player, like sharing a funny story
Regardless, keep up your normal pace and shot routine
“Remember that someone else's on-course meltdown is not your fault, so don't feel guilty about it. Make your best effort to play the best golf you can,” says Eleftheriou.
After the round is over, Eleftheriou suggests sitting down with your buddy and explaining to him why his anger management issues or habitual slowness are creating tensions.
"Consider taking time after the round—perhaps over lunch and on a different day—and let him know you’re just not having any fun playing with him any more," Eleftheriou said. "In a non-judgmental way, describe what’s happening on the golf course and how it’s affecting everyone. Then listen and give him time to respond.”
There may be other things going on in this individual’s life which is causing these reactions. Or, they simply may not be aware that their behavior was causing tension with everyone in the group.
Consider sending an email if you find it difficult to speak face-to-face. If this person is someone you would enjoy playing with again, provided they can manage their anger better or speed their pace of play, then it is well worth the risk of speaking to them.