Yes-maybe she could try harmless disencouragement of this "outsider" as his behaviour to me suggests he feels threatened
When this other cat comes in the garden throw some water on him/her a couple of times and hopefully this will stop him from coming into your friends lads territory-i mean just like most men-he (your friends lad) does have his pride and as he is pipless and he can't go out and "romance" the intruders gals or drop paw him-he needs it too
