Yes, I agree with you. But if the marriage went to the tribunal and was not declared invalid, one should act as though it is valid.
Why did your friend remarry? Is he Christian? Was it a Sacramental marriage? If so, why was he allowed to remarry?
Very true, sometimes the mentally ill person is separating, physically abusing, committing adultery over and over.
But sometimes a spouse tries to have conjugal relations after making mistakes, and is denied for long periods (a whole year) and does nothing to help their spouse get help. It’s not always a Saint married to a monster. There are things going on by both spouses.
As long as efforts are being made, out of love, to help and hope for a stronger marriage, then that is all someone can do.