Lol, shouldn't warn people for joking around with their buddies.
Context man, it matters.
However in group chat it can be quite hard to see the context when you're not part of the conversation, therefor Michael should be spoken to properly about the situation, before being warned, and shouldn't be warned for joking around with a friend unless his friend took offence to it.
However I will not jump to conclusions here as I said, context matters, and I can't grasp the situation to it's fullest since I were not there when this went down.
Good luck with your unwarn however.