Conversation about friends to friends is social grooming. It's our civilised version of what apes and monkeys do when they comb each other - it produces bonding and a feeling of belonging.
Talking about friends is icnluded under bonding, but being nasty is something else eintirely.
A good measure of the leve is to imagine the subject sitting in earshot - is what you are saying something that would hurt them, or make them angry, or something that you have heard, but can't know if it is true? Then it's bitching.