Q: Why do you assume that "pushing her into OM's arms" is necessarily a bad thing?
Right now (and especially since no one knows of their affair), they get to enjoy all of the romance, mystery and intrigue of their illicit relationship, unencumbered. It is a FANTASY they are living, and they THINK they have found their "soulmate."
We all know that soulmates have bad days, and foul moods, and bad breath, and kids that are underfoot, and body odor, and money problems, and . . .
Well, you get the idea. Just food for thought. There are a ton of reasons why a betrayed spouse should want to keep a wayward spouse in the marital home with them (her own personal safety, being able to "live out your changes" in front of her, etc.) . . . but "I don't want to drive them into each other's arms" shouldn't be one of them, in my opinion. Often once they see each other as they truly are -- warts and all -- the fantasy fades quickly. ESPECIALLY once their affair is out in the open (please note I am notadvocating exposure here, as that is against DB principles).