If this woman were to make the decision, and follow through with her actions, to step away from the (technically) adulterous relationship until and unless he divorces his wife and marries her... and the man in question decided to ABANDON HIS CHILD because of that... I do not see this as being in any way the woman's fault. I would see this as evidence of what this man in question has already demonstrated: he's only involved in the lives of his children insofar as it is convenient and beneficial to him. Otherwise why did he let his wife run off with his older kids in the first place?
My ex-husband/kids' dad loved to play at being the super-involved dad... when it was easy. But he's never ACTUALLY placed the kids' needs, or his responsibility to teach them and raise them and develop a healthy attachment with them, above his own wants and whims. He up and moved 800 miles away from them for cheaper rent and a better social life. He loves to pay lipservice on social media to the importance of fatherhood, dads' parental rights, etc.... but in reality, no matter how many times he says "omg I miss them so muuuuch" to anyone who will listen and fawn over what a wonderful father he must be... he hardly ever calls to talk to them between visits (which are months apart). Buys all sorts of expensive crap for himself and pays as little in child support as I let him get away with. No REAL EFFORT above and beyond the bare minimum. He talks to them when HE NEEDS an "emotional fix," and that's pretty much it.
Based on my own experiences, I would not advise this woman to give much weight to the possibility that the man might abandon her and their child if she insists on taking the appropriate steps to make the relationship legitimate.
Is that kind of man worth taking any exceptional action, or making any substantial moral compromise, to keep around in the first place?
I don't think so.