I'm going to tell my friend not to move in with another friend
Why don't you want your friend to move in with anothe friend?
That other "friend" proved themselves to be a duplicitous asshole. I don't want them to hurt my other friends.
While you cannot stop them from doing what they want, you can talk to your friend about your concerns. It will at least let him know that you care.
Nah, I'm going to threaten to stop talking to him so he will listen. I tried talking to him but he never listens to me.
I think that's going too far and will just drive your friend into the arms of this other person you want them to avoid. Backfiring, in other words.