Okay lets say that happens. Not a lot of people are gonna be willing to fight a long and expensive legal battle. But you can spin it however you want. They can just start treating him poorly. And those things are hard to prove.

There are places with at will employment so he can be fired for pretty much any reason. Also they can say no he bad mouthed the company.

The point is if his other company oportunity fails and he gets fired then he is jobless. That is what they are worried about. If he gets the other job no big deal. But being jobless can really suck depending on the industry.