I don’t think you’ve had a battering, OP, but I’m sorry if you’re feeling got at. I think posters (like me) are trying to be practical, but not aggressive in any way.
I’ve been through teenage years with three children, one of whom was bullied and another very much was picked on and excluded when a new girl joined the group. I learnt to set the bar for interference higher as getting involved backfired more often than it helped.
I also learnt to separate Bullying and ‘being nasty’ or making life unpleasant. One means get the school involved, the other is “help to navigate”.
When dating within a friendship group, the fallout is always awful as people take sides and previous social alliances fracture. Learning to navigate this with some grace is a very useful skill but it involves a lot of missteps and upset on the way.
If telling your friend you expect her to get her daughter to “butt out” works for you, great.
IME it just sets adults against each other and they hold grudges about hurt or criticised children LONG after the kids have patched things up.
It’s also very , erm, blunt language which also can backfire.
Very different to speaking to a friend and asking her to have a quiet word as some posters have said.
Stirring isn’t bullying unless it crosses thresholds like @OfficerChurlish specifies. It’s a term bandied about pretty freely. I don’t think that is helpful in the long run.
I hope your DD finds a nicer gang of people to spend time with or that things settle down in her current friendship group as school restarts.