Moei, I think you get too wound up into "stuff" around you. First he say he has a great personality but he's a bully in the pen to your mom's horse. Then he chases the other horses around during feeding but yet you say he is separated. I would almost be happy he is out of the pen with the other horse. And personally, I don't like herd bullies much. Just not my favorite trait in a horse's personality.
Honestly, I think you should just worry about your own horse and not get so caught up in things around you - ie who's talked to who and what's been said. If it's so bad, the BO could ask her to leave - with the horse. Maybe it does make him faster? Could that be what's really bothering you??
Let her be with her horse.