Wait till he comes back, sit them both down to a meal in a public place. Eat, chit chat and calmly tell them both what you think and feel and what they have said to you - cheating, ring, etc.
Dump it back into their lap to think about and let them make their own decisions.
Oh and leave after you've told them, so make sure to take your own wheels.
Note: I know this sounds to be a bit uncaring but people LIKE to make their own mistakes and as much as we look back upon our own mistakes wishing someone would have stopped us; we would have make the mistake sooner or later anyway.
Dump it back into their lap to think about and let them make their own decisions.
Oh and leave after you've told them, so make sure to take your own wheels.
Note: I know this sounds to be a bit uncaring but people LIKE to make their own mistakes and as much as we look back upon our own mistakes wishing someone would have stopped us; we would have make the mistake sooner or later anyway.