The problem with this is...
Well, okay, I have enough confidence in you to figure out what the actual problem with this is. And nothing more is needed to be added to be added to that. But there are other problems that will only make Donald Trump look stupider and stupider, assuming he actually is going to sue, and not actually trying to make a public issue of not being the son of an orangutan.
For starters, there doesn't appear to be any agreement of a bet between Maher and Trump, so it's hard to lose a bet you didn't make. Also, you have to prove damages. And considering how clownish Donald has come across for the last decade, it's hard to imagine he's suffered any personal loss -- especially since it presumes that Mr. Trump would probably have to demonstrate that there were some people who actually believed him to be the son of an orangutan.
Mainly, though, since it's clear that Donald Trump doesn't understand comedy, he doesn't understand that the biggest dream that Bill Maher has may be that Donald Trump would sue him. It's not just that he now has comedy material for the foreseeable future, mimicking all the Obama Birthers, claiming that the Trump birth certificate is a fake and that there's a conspiracy to suggest otherwise -- but I suspect that Maher might even pray for this to go to court, since the concept of expert witnesses swearing to the possibility that Donald Trump could be the son of an orangutan is comedy gold. And any financial cost of paying for lawyers, would be far off-set by ratings leaps, and besides -- since it's all a part of his comedy -- it's all tax deductible. And for all we know, HBO will happily be paying for it.