I'm not judging, I get every situation is different. It sounds like you're referring to a specific incident, 10 year+ relationship, things change, spouse is a drug user.

Sounds awful. My opinion is still that one should leave the relationship ASAP, regardless of how difficult it is financially. Support groups, family, part time work, driving uber, whatever it takes. I think it's unethical to stay in a relationship for financial security while trying to find a replacement.

Even if there was emotional or physical abuse my answer would be the same. Exit the relationship, find support, don't have an affair.