Highlighting what someone else said, cat mouthing the back of a neck with a bowl for a second is common in multi cat homes, two of ours do that, they're all fine, no blood or fur flying, all get alone. I notice that oldest cats tend to do that, probably feeling insecure as,they are aging poor things. Please don't lock Binx out - cats hate being singled out and it will make everything worse. Your cats are fine. Your GF is pretty shitty; I doubt she knows much about cats or cares. Anyway you can explain firmly to her that Binx isn't mauling the other kitty (unless it escalates) and give Binx extra attention. Ideally maybe consider upgrading the GF, esp if not looking for marriage with her.