I think men and women can be just friends in certain instances. Though I will admit 98% of the time one of the two "wouldn't mind" dating the other. I have a really good platonic female friend. We've been friends for 20+ years but I know per my Spidey Sense that she would date me in a heartbeat, so I'm careful not to lead her on. Still, we have fun when we go out on a platonic level. My gf of 9 years had a male friend she would hang out with regularly. He was maybe 17-18 years older, a 2/10 in male attractiveness and quite beta. Put a gun to his head and he probably would have said he wanted to date my ex but I perceived 0% threat so it was no big deal to me. I actually liked him and we would talk too.
So, a few moving parts here for your friends situation:
- Is the relationship a LTR? If yes, then move on to next step.
- Can the other male be perceived as a "threat" to the LTR? Looks, chemistry, $, current LTR on shaky ground, etc.
- Does he even care? With my ex and every gf, I carry an abundance mindset and for good reason, so if she f*ukcs the guy or gets distant or bitchy, it's good-bye.
If it does bother your friend, I would simply come straight out and tell her he feels it's inappropriate. Tell him to be a man about it and men call out bad behavior and BS.Then, when she plays the "you're just jealous card" I'd say "Ok, this isn't the kind of relationship I want, but good luck to you" and leave. I'd tell her to call me when she changed her mind about things and go NC. Leave her hanging. If she persists, tell her you want spend time apart. If she tries hard to get back after a while, do so under certain terms--tell her no more f*uck boys. If she stays NC, then your friend has his answer and he moves on. 4 billion women on the planet. He'll find another. Just tell him NOT to make a big deal of of it, like he could care less.
Good luck.