I would suggest that you speak to your priest. In the Roman Catholic Church there are, in fact, different degrees of sin. I'm no theologian, but certainly stealing to, say, feed your starving children is less of a sin than cheating on your spouse. Thus, the wording of of the applicable Canons use the modifier "grave" in front of the word "sin" because some sins are in fact worse than others.
It's also telling that the woman went to the the unusual effort to confront the priest in the sacristy - where no one but the people actively involved in conducting the Mass should be - while he was in the middle of preparations for the Mass. That shows that she was quite obstinate in her sin.
Finally, if she went to that effort to tell the priest it seems pretty likely that it was "widely known to those present at the Mass."
I initially thought the priest had gone too far. But when I read that this woman entered the sacristy to introduce her lover it became clear that she was not victimized; in fact she deliberately set up a confrontation, using her mother's funeral to make a political point. Despicable.
Not just in the RCC. Scripture talks about different levels of sin ... deadly and not deadly. All Christians should be in line with this teaching.