It's difficult to understand what is going on here without more information. What do you mean by scared? Is the horse scared of the other horse when it is alone in the field with it or is it only when your friend is there? Does your friend wave her arms at your horse every time she goes into the field? If so, I would say that your horse has never understood her intent which is to give her room to catch her horse and get out (I suspect he crowds the gate to get out?)  Does your horse get panicky when her horse leaves. Is he alone in the field when the other horse leaves? Do you ride out with your friend and her horse, how is your horse then? Is this still a problem for you?

 Did you ask your friend if she's been scaring her?