A bully does this because they can. A bully gets away with it by selecting their victim. If they tended not to get away with it, they would stop. If they do get away with it, unfortunately they are likely to do it again. I doubt he would see himself as a bully but that too needs to be brought home to them when it does happen. Usually if a crew member was brought in they would attempt to de-escalate the situation but they won't go along with the notion that anyone owns a particular overhead. So yes, ideally what your mother would have said is "shared space, kindly speak to the captain / inflight manager as to your options".