My boys are 19 and 20. This friend has been part of their friend group since junior high, which was before he expressed the problematic thoughts.

They aren't really friends with him at this point. I don't think they're afraid of him physically, but he has access to weapons. He's the kind of guy who carries knives "just in case".

I think they mostly see him as unstable and don't know what he'd do if they spoke up, but I agree with you - simply saying, "Yo man, not cool" would probably go a long way. But they've chosen to let the friendship fizzle out instead.