Tell me why it is he should continue raiding with them if he's on the lowest loot priority? It sounds like he disputed what was a bad call. It also sounds like, dispute or not, it'd have been a wipe due to said call. I've called over raid leaders in the past who've made bad calls, and we've downed the boss because of it. The raid leaders didn't sulk about this, they thanked me for picking up the error fast enough to save a wipe.
If you're supposed to be working together, you should be taking people's input on board. If you make a mistake, someone moves to corrects it, and then you react towards them in a hostile manner, I don't blame the guy for bailing.