Agree with Akasha. The sad thing is, when human beings are involved in anything, there is no way to predict an outcome because most of them are fucking nuts.
At my dog park we have a sort of "no ball, no toys, no food, no stick" rule because of the kind of thing you experienced. On rare occasions, like when the park is very uncrowded and the dogs there are very well known to each other, a game of fetch MIGHT be ok. Once, there was just a retriever, some type of (super cute) hound, and Hari. So we played a big ass game of fetch. But all theses dogs had played well together before and had no issues with resource guarding related aggression. Once another dog showed up, though, we put the ball away. It just doesn't make sense to take chances.
I hope you girl is feeling OK and that the other owner is as understanding as you!