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Can she stay with family or friends for a little while? She needs to be away from the cheater to clear her mind and make productive decisions toward how she wants to proceed. Men will always act like they didn't really want the other woman, especially when it's no longer a juicy secret. That's the problem with cheating, too much of it is for a thrill of "having something extra" without anyone knowing. Once secret is out, people sing a different tune.
Does she have kids with the husband? If so, that makes it difficult, but I don't believe in staying with and suffering potential crazy consequences mentally and physically from being with a cheater. The way I see it, kids may be disrupted, but the adult has a life they need to set right for themselves too.
I feel bad for her, as a married woman, cheating = divorce for me. However, if she's been married long enough and wants to leave him, it would be wise to not have any/more kids with him and look at how long it takes for her to dip into certain assets. If you watched that Huntsville show Melody found out that 10 years of marriage is enough to dip into spouse's retirement savings. I don't know if that's everywhere, but also the fact that he cheated may entitle her to more in a judgement. It's important to have a plan to lose that loser in the right way if she plans to leave.
If she wants to stay, I think he'll have his cake and eat it too, creeping out and using that child as an excuse to see the other woman. Also she will have a spouse who pays for someone else's child, because the side woman definitely still will want her child support. Paying for someone else's child takes money out of their combined household income. Essentially, she'll live for 18 years with her money (because his is hers) being used for someone else's child. That's going to be a no for me.
I don't like bluffing, so if he was really about that lifestyle with the other woman, then leave him. Let him enjoy life with the other woman and the new child, because of course these women always want to keep these loser's seeds.