I think its less about morals than it is about maturity.
I often interpret maturity as the willingness to do something uncomfortable simply because it will lead to a benefit for someone else.
Most people aren't willing to confront him and experience that discomfort to save the marriage at all. That's normal. What you can ask them to do as an alternative is to refuse him access and to tell him OW will never be welcome in their home. That's easier than confronting him... and less uncomfortable.
You do what you can Antonia. In laws are always a problem. You have to divorce bust them along with your spouse in most cases. Remember your spouse is leading a campaign to legitimize the situation, You just have to tell the truth.
If you say nothing, they will believe and support him.
