The wedding vows Mel took and purported to believe in didn't contain a contingency for declining quality of companionship. he's a hypocrite.
What if his wife has abandoned him?
What if she had an affair, and wished to reconcile but he did not?
What if she had an affair and enjoyed it, and wanted to be with someone else?
We don’t know what’s going on in their relationship. She may have pulled the trigger, not him.
The fact that he has an attractive new companion in itself isn’t evidence he is in the wrong. It’s not evidence he is in the right, either, to be fair.