How about something like
“They’re both adults now, should be more than capable of making their own arrangements and honestly my dd no longer feels a particular affinity with your dd, they’ve grown apart”
But if she pushes it - I’d be honest!
She can’t help her dd if she doesn’t know what the issues are.
You say there was good reason implying her dd did or said something out of order, well the dd is an adult and loss of friendship is one of the consequences of saying/doing stuff that annoys/offends people, time she learned to be frank.
She tried to split up my DD and her boyfriend so she could go out with him. ooft! In that case what does her dd bloody expect?!
I’d be putting friend in the picture
I've ended up friendly with my now adult dds best friends mum we have very similar parenting style and opinions.
The girls have at points fallen out and they soon learned that we wouldn't be dragged into their squabbles BUT also that they wouldn't get away with lying to their respective mums if they were at fault as between us we'd work out the truth of the matter!
We've both (other mum and I) had occasion to say to our own dds "well you can't treat people like that and expect there to be no consequences. You want things to change you need to apologise for what you did and not do same again"
Chasing someone else's guy is shitty whatever age, my dds strong small group of friends (7 of them now) have never accepted such behaviour, there was one girl who used to be part of the group who did this, she apologised the first time and claimed to be unaware of the existing relationship but the 2nd time she tried that nonsense on she was ousted from the group - consequences of her own actions as it was clear she couldn't be trusted and wasn't a loyal friend