BonsaiK said:
Imagine if you had a relationship with someone going and you loved that person, and a sibling interfered and without even consulting you to ask you how you felt put your lover in the hospital because it was "in your best interest, because I saw her cheat". How would you feel? At the very least you would have probably liked your sibling to sit down and have a calm, rational chat with you about the situation before taking action.
Again, you're focused solely on the question of whether the OP should harm his former friend
because of the cheating. I'm saying I agree with you on that count. The OP should not intervene "for" his sister. But he can certainly put the guy in the hospital for his breach of friendship and honor. You're focused solely on the "sister-boyfriend" relationship, ignoring that the "OP-friend" relationship came first.
I completely agree with you that there should be a segregation between the relationships. The OP-friend relationship shouldn't influence nor be influenced by the sister-friend relationship. But, if you accept that, stop trying to conflate them. The OP can beat the unholy hell out of his former-friend, and for reasons entirely divorced from the sister-friend relationship.
If my brother beat the crap out of a girl for cheating on me, I might object a bit. If he beat the crap out of a girl because
the girl mistreated him I would have no cause to object (as you've stated)