Given the title you used for the thread, I assume that what you are saying is that your mother is blameless, and so the law should not apply to her. I would caution against this approach. St. Paul says that the law was made for sinners, but if you go a verse or two further, you will see in the list of things that the law is meant to combat: “whatever else is contrary to the sound doctrine 11 that conforms to the gospel concerning the glory of the blessed God, which he entrusted to me.”
That is, the law serves truth. Your mother was clearly wronged. But entering a second marriage while still married is simply impossible regardless of this, and to attempt to do so is “contrary to the sound doctrine” that St. Paul references. Don’t misunderstand - I am not attempting to condemn your mother. There is no way whatsoever that I could begin to have any idea what sort of culpability for attempting a second marriage might exist. Nevertheless, attempting a second marriage while married is objectively wrong, and the Church must act in subordination to this fact (though, as 1ke mentioned, there are ways to regularize the situation).
No one condemns your mother, but the Church has a duty to teach the truth, even when it’s hard, and even when the unjust actions of some unfairly burden others. So while we can all see that your mother was wronged, we can also see that some things just aren’t possible, and that the Church is simply unable to act as though they are - no matter how much we sympathize with your mother.
But again, I want to reiterate what 1ke said - there is a way forward. The Church recognizes that your father wronged your mother, and teaches that your mother’s response, while likely motivated by the best intentions and certainly a sympathetic case, was also wrong - but that there is a way to correct the situation.
