I've seen a plethora of responses for problem players, but my buddy's style is one that I'd like to get more specific about.
The issue is his play style. Our DM and I have been trying to work with him on his character. Now, in our same campaign, he's played a Rogue character that really didn't gel with the rest of our party. We were okay with this because that can be interesting if you play it right. But my buddy is atrocious at it. He included a ton of metagaming, out of character choices, and bizarre combat decisions that were totally unnecessary. An example would be announcing "I nock an arrow". He's not readying an action, he's not trying to be alert, or making an attack. Then he'll try to jump over a barrel, throw a knife, then shoot two arrows. Which our DM will respond, "Bro. One movement. One action". It also became clear that my buddy didn't understand how the class features worked. Whenever we try to help, we get scoffed at, because 'he's got this', 'he knows what he's doing.'
After a journey of attrition with the character, my DM decided to kill it off, just out of mercy. The idea was to help my buddy make a better character, one that he knew how to use. And one that we could hand-hold him with subtly. Now he's doing the same stuff. My DM is getting ticked off and is thinking about kicking him out.
We're just a group of friends that love to play together, and it'd suck if we lost my buddy. I think it's long-term best interest to help him get better. He's so prideful it's hard to confront him. How can I approach his behavior and help him?