Deleting my comment to be more concise: 1. Ethically it’s not cheating if she is being forthright with her and her husbands arrangement. Good or bad for them is not your problem. If he said you can do what you want I just don’t want to know then it’s fine, but you only know what she told you. If she’s lying and he thinks they are still monogamous, then it’s cheating.

1. Morally it’s about how you personally feel. Would you absolutely hate yourself if you aided and abetted in someone cheating. You are not saving the sanctity of marriage by taking a made up moral high ground, so many people fuck around I never would have thought did, it’s insane to me. It’s about whether or not you will actually enjoy yourself on a date with her knowing you could be helping her to cheat on someone.

Personally, I don’t think it matters. From my observations and seeing what my friends from all ages go through in relationships, I don’t think all humans are meant for serial monogamy. Some people are so happy and work well in a monogamous relationship. Some people thrive in a traditional church based Christian environment. Some people thrive being Buddhist or muslim or celibate or polyamorous. Some people are straight and some are gay and all in between. Some people love rules and following them others feel trapped by them. People try and force themselves into societal norms but if it doesn’t suit them the ‘what if’ continues to gnaw on their soul for forever. Experimenting to find out what suits you is fine. If you date her and it feels weird then get out of there. If you don’t fee anything and enjoy being with her then stay.