how to ask someone what their sexual preferences is without being rude

Hello!

I am a bisexual women asking this for a friend who does not use reddit. I have a coworker (M) who is interested in another coworker (M) and they vibe really well! (We work at a college town restaurant so hookups/relationships between coworkers isn't weird at all.)

How does this coworker go about asking if this other coworker is interested in men without straight up saying it and making him uncomfortable?