Oh come on, she's been rumbled.
Her Dad is the coach, she thinks she can get away with murder!
It sounds like your DD has been throwing her weight around thinking she can act with impunity because she's got her Dad in her corner.
It's not fair. DH needs to stand down from the team as manager and someone impartial take over.
Of course DDs not going to admit it, she's been caught and she's embarrassed. If others can corroborate the slurs, then you have to admit she's been mean and she needs to stop.
And of course it needs to be dealt with impartially with an outside person. If it's your DH and other parents it will just turn into "she said, he said" pointing fingers. I wouldn't go down that line,.
But most of all your DH needs to coach a different side and all the girls will feel like they are equal again.
Most parents who are teachers these days deliberately don't send their children to the school they work at for precisely this reason. They are too open to critisicm for potentially unfairly favouring their own child.
It sounds like this may have been happening - even if your DH is an unwitting partner in it all. She has felt safe to criticise others, thinking she won't get into trouble.