The younger ages is where this is usually an issue. As the girls get older they understand social dynamics better.
What I tell every team at the very start of the season is there are two things I absolutely demand from them and there is no grey area. The first is they WILL respect each other on and off the field, especially on social media. I make it clear to them I don't expect them to be BFF's with everyone on the team, and that's OK. It's part of working with a group of people. There will be differences.
The younger teams need an absolute set of rules sometimes so they understand the boundaries of what is acceptable. On the rare occasions, so do some parents. Being consistent with rules is what the HC needs to do. Kids like rules. So long as they are fair and handled the same for everyone.
I wish my DDs HC did something like that. Oh well, she is young and inexperienced.