I'm going to start by saying that I think that apart from one or two posts, this is the most thoughtful, sensitive, most sharing and interesting threads I've ever read here. So
to all of you. When I was reading through the thread I started scribbling down names of posters I wanted to comment on (in a complimentary), and there were so many, I had to stop!
So there is some fantastic sharing and advice on here.
My suggestion is to say to your friend "how do you feel DC are getting on at the moment?". This gives her the opportunity to tell you how she sees it from her perspective, and will be a massive help to you in deciding how to move forward. I.E. If she denies any kind of a problem ("they're best friends, aren't they!") I would seriously consider dropping her. Any other answer can lead you into a conversation which will make your friendship closer and support each other (maybe she thinks you're being precious and over-protective about your dd? Maybe she's afraid of confronting her son, etc). An honest conversation - (and not a mean one) could be brilliant here.
But there's something else I really want to say, and it's no disrespect to those posters (whose names I've scribbled down, but can't recall who said what now!). I TOTALLY get those posters who've said "I wish I'd listened to my child" and "my child felt unsupported because they reported things and I seemed to ignore them". YES. I get that. And how huge it is. We MUST - all - listen to our children.
BUT we all must equally NOT get sucked in to kiddy stuff. They DO blow hot and cold, they do get upset and then forget about it. They fall in and out of friendships, etc.
Not so much in this case. And I would NEVER tell a parent to NOT listen to their child. But please, please, please be aware of getting sucked in. Me and my DSis are no-contact for entering our third year over what I now know was a row the kids got into. I didn't see it as that at the time, and my DSis won't forgive or forget.