Will try not to waffle...
Ds is in Y5. Happy, plenty of friends, esp 2 boys been friends with for ages. Usual thing of 3s a crowd sometimes but no big deal. One would often play with others etc. will call them A and B.
Now another boy, C, appears on the scene as his bf left the school. Great, a foursome. All seemed lovely. Then ds came home telling me A and B don't want to be his friend etc etc. and they have been arguing a lot. Completely ignore each other or having really nasty arguments.
Sad ds but can't seem to work out what problem is.
Today, a completely impartial party said her dd, who has no hidden agenda or anything to gain by saying this, said that C had been telling A and B all sorts of lies about ds, so they wouldn't be his friend because he, C, wanted ds all to himself. Well it has worked because ds really likes C, they get on so well, and he feels very happy that he was there when A and B fell out with him. Seems that C was the cause.
I feel really really 
for ds because whatever his faults, he is loyal and I never hear him bitching. I just feel incredibly let down on ds's half, feel that a situation has really been manipulated. Ds isn't especially happy at the moment because of all the tension amongst his old friends.
Please can someone help me, advise me on how to handle this. It has really really upset me but I know I can't be upset when I see ds as that won't be helpful at all.
Thank you.
PS I'm a regular, just name changed for this.