I don't like the idea of presenting this as a decision... It seems to dignify this affair... Is that what you want her to think?
and rather than "soon" how about... End this or I will end it for you...
It IS a decision, and he can't make it for her. All he can do is communicate his own Boundaries of Personal Integrity ("I will not live in an open marriage"), and let her know what the consequences are if she violates them.
I think it all depends on whether you're going for COMMITMENT, or COMPLIANCE. Me, I'd rather have commitment.
