What should I do about my friend's possibly bi boyfriend's antics ?

Eh, I wouldn't tell the girlfriend in hopes of her controlling his lust. She'll be mad he isn't be truthful, they might break up, and then he'll come at you full force.

If he isn't out, you shouldn't tell at first on principle. I just don't like when people out other people.

Is there a way you can solidly reaffirm your feelings? Make it click for him that you aren't interested?

If he does it several more times even after you tell him to stop again and again, then I suppose it would be ok to tell the girlfriend. Even though I don't like outing others, I FIRMLY believe that if you're in a relationship you shouldn't be seriously flirting or hitting on others.
He's a little slimy, so it wouldn't be absolutely terrible if you were a little slimy to protect the girlfriend. Your loyalty is to her after all.

It's a tough situation he shouldn't be putting you in.

 

Give him the opportunity to stop, in the interests of allowing him to remain closeted - outing someone is not something to be taken lightly. But if he persists, then I think your friend needs to know.

By 'give him the opportunity', I mean tell him to stop or else you'll tell.

 

Cheating is cheating. Its not so much about "outing" someone as it is about doing what is fair. Years later at the end of an abusive relationship imagine yourself saying, "Yeah i knew about his cheating a long time ago but i didnt want to hurt hiz feelings so I never told you."