How did the friend die at the end?

Although it's not entirely explained, given the few moments of exposition regarding Tommy's inability to swim, I believe the assumption that he drowned is entirely sound.

It's not important. He's got nothing to fight for, sooner rather than later he'll die drunk anyway for whatever reason. That's the point of the movie, alluded with the Denmark national songs they sing repeatedly.

He had lost his wife and his career, and his dog was near death. He had nothing to live for. He might have fallen off his boat in a drunken stupor and drowned, or had a heart attack, or drunk himself to death. He might have committed suicide. Exactly what happened to him is never revealed.