The problem is not the child herself, as a person. There are marriages here where they are raising the OC - the problem is that your marriage won't recover while he spends the next two decades in continual contact with his mistress.

If you can't handle him making that choice, then it really is choosing a marriage where you are always in danger of him running back to her. Death of a thousand cuts.

She has nothing to lose by trying to win him back since he essentially wants to keep two families.