I would almost be willing to bet that as soon as your friend & Dan break up that Boss Hogg (good one Jill!) will move on also -- he will no doubt not want to have a girlfriend who has that much free time on her hands and no worries of her own to concern herself with about the cheating that is going on from both ends. As long as she has a boyfriend of her own, she has something to lose, just like him, once that is no longer part of the equation, I'll bet he will want nothing to do with her.
Again, the kids are the ones who are going to pay for all of this in the longrun - how do you think she will feel when that one is thrown in her face.