When you're in the pasture, you
are herd leader. She's not allowed to disrespect you. If this mare tries to run off your mare, go out with a lead rope and pop her with it if she tries to get near her. After a few tries, I'm sure she won't try it again.
Other than that, as long as your mare isn't getting bit up/injured out there with her, she'll be fine. I don't think that a horse will just mercilessly pick on another one for no reason
. Anyway, she's moving out soon anyway, so problem solved!