I think Hopkirk's advice is excellent.
Friendships ebb and flow and a golden rule for any and all parents, is to stay out of it as far as possible.
There is no good way, and no good outcome, in telling any parent that their child is not behaving well. They may know that it's true, they may not, but even if they do, they will not take kindly to having in pointed out by another parent - the automatic response is defence at what they will see as criticism of their child, and their parenting skills.
The easiest way is to avoid the relationship angle altogether, and stick to vague uncertainty.
The result is exactly the same, but the door is not slammed on future friendship repairs by the girls themselves.
And equally important, the door is not opened on future frosty relations with the daughter's friends' parents either.