They could've claimed he broke his employment contract (I'm assuming he had a contract - he almost certainly did) which presumably had some "bad boy" clause in it, thus firing him for cause and not paying him anything. But that is messy, and costly, and the outcome is uncertain, and while Rubin sounds like a scumbag here, this also wasn't the most egregious behavior out there. Bottom line, Google was screwed the moment they signed that employment contract...those things are written incredibly favorably for the employee.