I have to add my 2 cents in here. I would say ABSOLUTELY NOT UNTIL SHE IS G - O - N - E. He's cake eating if he's living w/ and having S w/ her and also having S w/ you. He then has no reason whatsoever to decide one way or the other -- stay w/ her or come back and work out your M.
Don't regret that it happened that one time, however, I would say get a chastity belt (and I'm saying that lightly:) until he decides he wants to work on the M and prove it that he loves YOU and misses YOU. When he says that is what he wants, there needs to be no contact w/ OW. I would say no sex until he has agreed to those terms.
Me: 38 H: 35 S4, S5, S10 Bomb 01/07 Wanted D - nothing would change his mind Numerous A's prior to D bomb; EA prior/during D bomb Piecing 04/07 Deployed for a year 05/07 Still Piecing 2010 M 11 yrs 05/10