That is part of why speech is protected. I very much dislike both Trump and Hillary and I've said more than a few times that someone should just shoot them and the world would be a better place. That isn't sarcasm it is just exasperation, I would never REALLY condone violence and wouldn't support someone who did. Every day friends and colleagues say "I hate that x" and "I could kill x" and none of them actually mean it. It isn't that sometimes you don't mean extreme and inappropriate statements it's that 99.999% of the time an extreme statement is made the person doesn't mean it.
He doesn't need a defense. We are all two faced when it comes to work and home. He can be a racist bigoted prick in his head, if it doesn't translate into inappropriate comments and actions at work it is nobody's business. If he has complaints against him at work, his facebook could be brought out to corroborate and show consistency between the two for what that is worth but no, you don't initiate real action because of something someone said on facebook.