I hope I'll never have a 'best friend' who allows himself to get this deep into a relationship with my girlfriend. But then again, maybe I have an old-fashioned concept of friendship and caring. Respect your friend's relationship.
I see too many people treating startups and exec positions like regular labor jobs, as if they are just entitled to be there because they know what's up. It's a real privilege to be a part of someone's idea to that level of intimacy, and what a shame to let something so petty as lust to destroy that intimacy and opportunity.