My best friend confessed to cheating a year ago. Do I tell her husband?

I am just speechless.My best friend told me recently that she had a ONS with a work colleague. Stupid stupid woman! I am shocked as I always thought she had a rock solid marriage. They have been together ten years. She always speaks so highly of her husband and says she is happy in all aspects of their marriage. Infact they are in the process of adopting and are both very excited. So why did she do it? She says she was flattered by the attention and was curious.... I asked her if she was going to tell her husband but she refuses to. She knows that it was a one off and will never cheat again she says. She feels disgusted with herself for losing control... living with this secret has been like a cancer she says. But she would rather live with this lie than her marriage ending. So now she has put me in this really awkward situation! I socialize with the two of them every two month and I don't know how I am going to behave around her husband. I am keeping her secret alive and feel terrible. But why is it my responsibility to tell him? I don't want their drama as I have enough of my own to deal with. I feel really angry towards my best friend for telling me. I also feel she is a coward. Infact I have lost a little respect for her.

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