Before you read my post-please re-read GG's post, it's got a WEALTH of info you REALLY need to consider first.
IF AFTER you go through the things above, you decide you want to make it work with him-
FIRST- HE needs to study up on what POLYAMOROUS is and HOW TO CHANGE from a cheater to a poly person in ACTION AND ATTITUDE before it will ever be possible.
(this said, from a wife who cheated and is now poly, with the husband and the lover whom I cheated with)
SECOND- YOU need to study up on it and identify how you will have to change in action and attitude from a monogamous lifestyle, mindset, expectations etc; to a poly one.
THIRD-you need to have a SERIOUS discussion about the other woman. AND if the decision is that he's in love with her and wants her to be his other partner-THEN
SHE ALSO needs to do the above steps.
That said, I cheated for nearly 10 years on my husband with the other love of my life. It was painful for all of us. When I finally found out what polyamory was, I figured out that my problem was being naturally polyamorous (capable of being fully in love with more than one person simultaneously) and trying to fit into the box of monogamy that I'd been taught.
I expressed my quandry to my husband. He wasn't happy-but he'd already caught me several times in the affair and was intrigued (through his pain) at my sudden honesty.
That was nearly 3 years ago. I committed on September 25th 2009 to not lie again-ever to my husband or myself or anyone else.
It's been HELL. The first year my husband was a total mess, seeking revenge, wanting to punish my boyfriend and I. He made false accusations and true accusations and he didn't believe anything I said.
But, I stuck to my guns always telling the truth-even when it hurt me to admit it and when I knew it was going to hurt him.
The second year was me struggling becuase he wasn't being honest. He was lying about what he wanted, what he could handle, how he felt. I was angry that I was putting myself out there completely-and risking the most vulnerable parts of myself to him honestly-only to have him beat me down with distrust and accusations AND protect himself with lies.
The first half of this last year was much of the same.
Then, a breakthrough in March clicked for him-when he lost his girlfriend and potential lover, through his own lies (of omission).
Since March our world has started to spin gracefully together, with my boyfriend who lives with us (and our kids).
But-that is three years of HELL. Not a little struggle. HELL.
So, YES it can work.
No you can't control polyamory by limiting "who and when" someone falls in love. You can limit how a person is allowed to act upon their emotions-but that often backfires terribly.
No it isn't easy to transition from cheating to full honest, open, loving relationships with one another-it's harder with multiple people.
(feel free to read my blog if you want to see some of the ups and down issues and suggestions that I found helpful along the way. But, seriously-go re-read GG's post a few times-it's VERY profoundly necessary steps she laid out there)
"Love As Thou Wilt"