
If she is bullying him and he is just running away, I would interfere - give her a time out, every time. Believe me, bullying can very easily become a habit - Soli bullies River something chronic, and finds it immensely rewarding. Which, of course, means that it's proving an incredibly difficult thing to break her of - much the same as ragging her lead at the start of the walk. She finds it such fun.
River will tell her off (which makes it more fun, so she does it more, so on and so forth). But Opi will not; before River came home, Soli used to do the same to her and it made her very distressed, to the point that she hated going in the garden (as that's where it usually happened). She's just now happy about it, and Soli's been here nearly two years (River just over 1 year).
Putting him in his place is one thing, but bullying? Not acceptable IMO.