Parents - what do you do if your friend's child is rude and the mother ignores it?

It's really getting me down - especially as she is very quick to criticize my kids publically. I try not to go there anymore when the child is around but had to yesterday. He is argumentative and opinionated, besides hing and downright obnoxious. I left after five minutes when it became clear (once again) that his mother spends her time humouring his rudeness. He's 12. She's a good friend and I value her friendship but her kid ....