There's an interesting article on the difference between being rude, being mean, and bullying here if you'd like to look at it.
Basically the author is a therapist who works with bullying issues on a national level and her take on this has three levels.
Being rude - is inadvertently saying or doing something that hurts feelings or causes offence although none was intended and its usually a one-off.
Being mean - is done on purpose, maybe once or twice but not repeatedly.
Bullying - is repeated and has an imbalance of power between the people involved.
What you've described sounds like children being mean rather than bullying, with friends of both children stirring the pot but your son feeling upset and remorseful afterwards. It doesn't sound like there was a power imbalance or anything like that.
It actually sounds like a good chance to talk to him about how he and his friend both felt on hearing and saying mean things, how he knows not to do it again, and why the people passing messages were behaving hurtfully and badly too so it's a lesson in not letting other people encourage him to treat his friends badly.
That's not to say he isn't responsible for himself, but I remember this kind of thing at school and it's easy for all of them to get caught up in the drama of messages being passed.