Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...
"in sickness & health ..." - that's sickness for her. While she doesn't really "know" he did commit to her and that life.
My view would be more of if he feels the urge to be with another person then why wouldn't he simply divorce the wife? If it's due to being a bad guy - "she's sick and he left her" it sort of runs into "she's sick and he cheated on her".
I don't think there is an answer that will fit every circumstances and the person would need to make that choice on their own.
She is his wife...for better, for worse, sickness of health!
Alzheimers? Paralysis? Cancer? Drugs that make one impotent? Depression???
I think the best way to stay in the state of grace is to NEVER take another besides your spouse under ANY circumstances. Of course, that's not for me to be the ultimate judge, but you can't go wrong by being too right...