I usually love Cary, but this answer drove me nuts. Yes, you want to develop empathy for the kid, but otherwise this was a totally useless answer. I don't think there's a good way to directly address it with the friend. You might bring it up gently, saying something like,
"Friend, I've noticed that Child seems like he's hitting that natural phase where he's a little more of a handful. I know a great book that might help. Are you interested?"*
If the friend seems interested, then you might make suggestions or give them a good book on managing behavior. But if you're shot down, really your only option is to slowly back away from the friendship and try to only meet with your friend at an adults-only place, like a bar or something.
*Note, I know what this kid is exhibiting is not necessarily a natural phase. But that's a non-confrontational way of saying, "Your kid is a brat", even if it is a little white lie.