Whose fault is this fight? Who's in the wrong?

Oh, that is really sad that you were trying to be a friend, to a good friend of yours,being his ear and he didn't appreciate it and called you a name.Its also sad and disturbing, his girl(who probably secretly harbors some jealousy of you) did that to you. Sure you should not have slapped her,but she should not have been so mean to you either. Its sad, but unless your friend tries to make up to you,I would hang out with better people. Maybe Callum is just letting his hormones get the best of him(most guys think with their *****), and when she finally dumps him, he'll come back around to you. Then perhaps, you can go on from there. If he means a lot to you still, after all that has occurred, you can always continue the friendship -at that time. Also ,understand that when a good friend has a girlfriend your relationship won't be the same with him and you need to give them lots of space.