What Minxxa and Mono said...
And my two cents.
For me, it's moral to have responsible, ethical casual or recreational sex. But those two words are critical. I would not be involved sexually with someone who would be cheating on a partner. I meet the significant others at some point, openly, as a potential interest, and they meet my SOs, particularly my wife who is my primary relationship.
You are certainly not responsible for how he handles his relationship with his wife. However you are responsible for any relationship you have with him. Even for a casual fling, this is a terrible foundation. I would lose respect for myself. Would you? What about your husband? Being perfectly ok with non-monogamy may not extend to approving of you helping another man cheat. I would be concerned about losing the respect of my spouse.
And even if your prospective fling never tells her, never gets 'caught' - you would be in a relationship with her too. She just doesn't know it. Why in the world would you want such drama in your life? I urge you to run, not walk, from this situation.
As someone who's dated men and women, I've learned that men, bless their hearts, are easy. There are scores and scores of men out there who would LOVE, LOVE, LOVE to be your recreational plaything who are not married or committed, or, if they are, their wife/SO is hip, onboard, and totally down with the situation. In fact, she would love to have you over for tea and girl talk. (Ok, I exaggerate about the tea part.) In fact, to get interest, all you have to do is sign up on an online dating site as "female". Seriously, that's it. You will have no trouble finding potential casual sex partners among men. (Women-different story but cheating still not a good idea.) So, again, run, do not walk, away from this man. There is no upside for you.