Simon Brown vs. Maurice Blocker is a famous case of friends in real life throwing down inside the ring. It was a great fight, but Blocker's career never really recovered.
Narco Antonio Barrera and Jognny Tapia were good friends. Holyfield and Henry Tillman were good friend from the 84 olimpics and after they fought in the pros Holy was Tillmans best man at his wedding.
Joe Frazier vs Bob Foster "Why'd you knock me out like that? Sorry Bob but you tried to take my title."
Besputin vs Butaev. As you can see, it got very nasty, especially because Besputin was schooling him boxing wise. Disgraceful display from Butaev after the fight.
Infamously the Duivens (a bit more than friends). Also somewhat infamously Aaron Pryor while legally blind, is alleged to have fought mates/associates in his two 1990 fights.
I saw that fight. Butaev was still pissed off afterwards and wouldnt settle down. I wonder what happened there.
Too bad that Ken Norton and Joe Frazier never fought, they were sparring mates, both were trained by Eddie Futch. But no amount of money could put them inside of a boxing ring, both have said this when asked.