Out of curiosity what's the worse that can happen here if you appear controlling?
Your wife might cheat on you? SHe's already doing that
Your wife might divorce you? She's already planning that too
Your life might fall apart? Hello?
Your FEARS are coming true anyways... you are already bringing that about BECAUSE you won't control the situation.
Your trading control for her freedom is going to cost you yoru marriage.
OK, sharing this info with the OM's wife may not help... But don't hold back because you are afraid of her. If you think this is gonna go away on its own you are a fool. I've been there it does NOT GO AWAY, it gets a LOT MORE PAINFUL for YOU after this point... we are TRYING to SPARE you that.
What IS the OMW doing to "fix" this? Buying him flowers and chocolates?