antigone

I have a 5.5 month old Golden Retriever male, who's best friend is a 6 month old Australian Shepherd cross female. I have noticed that she is extremely territorial and aggressive towards food and toys. And when they play, it's not like how he plays with other dogs. She nips and bites him and generally terrorizes him. Her parents think that there is nothing wrong with her behaviour and that it's cute. I'm just wondering if I am being overprotective (she actually attacked him once, though they thought she was playing, but, to me, growling and a full on attempted fight, is not playing) or if this is a problem, and if so, how do I get her owners to realize how serious this problem could get. Also, she has already attacked another dog, and I am worried that she will do it again, or even bite a child.