Mediate when you can to help them solve the issue.
“You’re both my friends and I hate seeing you fight. Can we try to talk this out now? I can help.”
“I’ll help you guys sort things out, but I don’t want to get caught up in the middle. I’m not on either of your sides, I’m just trying to help both of you get along again.”
“I can see what both of you are saying. Can you try explaining it to each other the way you did with me?”
Lighten the mood to diffuse tension.
“You guys are both being ridiculous, to be honest. Can’t we just go back to being weird together like normal?”
“In about three days you’re both going to realize how dumb you’re being, and you’ll respect me so much for putting up with it.”
“Oh stop it. You know you both love each other. Just hug it out and let’s go get pizza.”
Remind them that you all care about each other.
“I know you care about each other. Once you get through this rough patch you’ll remember that, too.”
“It sucks to fight, but you guys are such great friends. I know you’re going to figure this out.”
If the fight is escalating, be serious and strict when you intervene.
“We’ve been friends for years and this is not going to be the end of that. Let’s talk about it, right now.”
“It’s not like one of you is right and the other’s wrong. You just have different opinions. Either agree to disagree or figure out some way to compromise and move on.”
Recognize when it’s time to back off and let them solve it on their own.
“You can tell me if you think there’s anything I can do, but I think it’s best if you guys try to figure it out on your own.”
“I’m not going to get caught up in the middle here. I know you guys can figure this out.”