I think I should tell my coworker that her husband is cheating on her but I'm not sure how to bring it up.
Why are you thinking of telling her about her husband's cheating?
I feel like I have the responsibility to tell her what her husband is doing. He deserves to be found out.
Cheating in a marriage is a very serious issue and can cause a lot of psychological harm. It is right for you to want to tell your coworker about her husband's infidelity so that she can make an informed decision to either continue on with her marriage or divorce. If you are having difficulty telling your coworker directly, you should leave her an anonymous note that can't be traced back to you.
I think it's going to be fine. I'm telling her tomorrow.
That is good that you are able to figure out how to break the news to her. I hope that everything works out for you both.