Not knowing all of the details, but was there any warning, a low growl or quick air snap. It not, it sounds like the other grey doesn't have bite inhibition, or an air snap was intended but a tooth got caught on your grey's ear instead. By your description it sounds like there wasn't a warning though. Often this means that a dog is punished for growling and when they are in an uncomfortable situation, instead of warning they go right for the bite.
I'd suggest they talk to their adoption group who may be more familiar with the grey's background.