Originally Posted by FehrGroundRanch
I would try to pull the arab out and let everyone else accept her and then put him back in. If everyone already likes her then he might be more willing to accept her.
That is the way I would do it also. The Arab should start to feel ostracized after a few days and will be less likely to show the same aggression.
Mares have a tendency to become the herd leader so don't be surprised that this may happen after a while.