I find sarcastic positivity often is good eg. "that's a really great idea - you should be president." Let's you take the piss and often they think you're being serious.

or just be professional and divert the topic to something else so you don't have to give an opinion on craziness. The guy was probably just mouthing off anyway and responding literally isn't going to do anyone any favours.

Exactly.

It's kind of a question of integrity. Do we have a moral obligation to disagree with something someone says that we feel is wrong? Even if it's not in our interest to do so?

Maybe we do, maybe we don't.

Personally, if someone with a large influence on my income says something I find offensive or outright idiotic, I disagree in a polite way that shows that I respect their opinion even if I don't. If I feel that it would really offend them I'm not above lying and pretending that I do.

Sometimes I feel morally conflicted about this, but it's kept me out of trouble.

I think that's wise. I could have done that. I actually sort of insulted him and that was probably unnecessary.

But if I had expressed an opinion explicitly maybe there'd have been a debate. Probably just should have said "I can't agree with that, but you are entitled to your opinion."