There are many qualities friends look for in friends, including being supportive, giving good advise, showing up on time etc. The willingness to defend a friend is one of these qualities. An enemy who values this quality of friendship very highly may start to see someone who stood up and defended another friend as potentially a very valuable friend. And go out of their way to foster a friendship after the fight ends.
Fighting friends who later on turn out to be friends in a minute sometimes happens unknowingly. At other times, the fight brings in respect between the victims who are involved in the fight although this is not the best way to make friends.