This sort of awful advice has led to the simping of men, and the marxist destruction of America: "you have no right to express your well-considered opinion. You are a good person only if you shut up, accept the bullshit, and deal with the mess afterwards!"
NO. You tell the man - you have already made a grave mistake having a child with this woman. I'm not going to stand by while you dig a deeper hole.
Did you read the whole advice? It said to speak her objections once, calmly, noting what has been observed; and do it in a way that leaves the door open for the day he may realize his error.
Nobody can learn another’s lessons for them, and you can’t force people to do the right thing.
That needs to be the father or a friend.
It appears the mother wrote in.
What does "I'm not going to stand by" mean? She won't speak to him? Or to her? Not invite them over or visit them? Or that she'll never stop nagging him about how horrible his wife is?
If this raging battle-ax's goal is to not have her son talk to her, or let her see her grandchild, then that's a good idea. Otherwise, not so much.
We’re continuing to support them financially, and I don’t see a light at the end of the tunnel. I’m no saint, my reward is seeing her improvement. I’m nearly 72, and have no expectations of ever retiring. My wife is a typical Hispanic woman, can’t say no to her two sons (my daughter is squared away), or to her family. After 40 years of marriage, it’s too late to walk away.
No tears for me, it’s just the way life is. Seeing my 3 grandkids is my reward. They all live within 30 minutes of me, it’s a great situation.
As to the OP, I am in no position to offer advice. Her son is an adult, it’s up to him to make the best decision. Having a child is a complicating factor, so do what’s in the child’s best interests. My parents divorced when I was 6, and I vowed my children would never have to experience that.