It seem obvious to me... keep communicating and now... why have you waited so long to say something to Ellen? If this were me I would of gone to Jim and told him so as soon as I realized I was interested. To me that is respectful and good manners towards someone else's partner... I would be explaining that it would be either me or him that tells her that he has cheated as I would feel uncomfortable holding that secret.
Now you are in a different spot because it has been too long a wait and he can easily put it on you because you didn't keep him in the loop and communicate.
When I wanted to date derby I discussed it with my partners and we talked it through. Then I told her husband and after I asked her for a date, she talked to my husband and partners. This to me is just respectful to those that are already in someones life. It's not a matter of asking permission, but just as heads up and gives some indication of whether or not its worth pursuing. It sets a tone for the relationship of caring and respect and shows good integrity I think.
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