Urge them to stop and warn them I'll be informing their partner of their actions.

Tell them I'm no longer their friend. If they can do that to their partner, what are they doing/saying about you behind your back?

Remind them what they’re about to do will have serious consequences whether their SO finds out about it or not.

Tell them that's custy as hell and ask them what's wrong with them, probably

Nothing, "cheating" is a stupid, immature concept and I don't care about it. Love isn't possessive.

I would fuck them, the only right thing for a friend to do:)

"You're about to cheat on your partner, are you sure about this?" If they are - tbh I'm gonna be disappointed in them (I don't understand why you can't just break things off if you're thinking about cheating).

I suggest to never see that guy again. If he can cheat on his gf then he should not be trusted.

Slap that boi silly tbh

Id say to him: "Handle nur nach derjenigen Maxime, durch die du zugleich wollen kannst, dass sie ein allgemeines Gesetz werde."

I might ask them if they're sure about maybe flushing their relationship down a drain, but what else am I gonna do? If I wanted to run peoples lives I would have kids