The mother is never punished in this situation. What are you talking about.

Divorce her. Kick her out. Whatever it takes to get her out of your life.

However, you're telling the victim of the paternity fraud, to suck it up and sacrifice himself...

Oh fuck off, no I'm not. I know multiple victims who, despite having been horribly mistreated by their significant others, still found ways to be their for their kids, regardless of blood relation. Some choose to walk away, and that's fine too as long as you're not basically leaving the kid for dead with a terrible parent.

No one is talking "sacrifice" here, and even if they were...fuck it, I'll take that over being that cold-blooded asshole who'd leave an innocent kid basically for dead with a terrible parent.