You gave a perfect reason for letting the boss know:

"I don't know whether he's being honest about not minding hiring a guy, or whether he's just saying that and he'll be a passive-aggressive douchebag to me from now on if I make an issue out of this."

This makes it the bosses problem. This isnt something you should even have to be concerned with.

Further, the coworker is already a douchebag for even putting you guys in this spot. Bosses need to know when people dont play as a team.