I (39M) am in a monogamous relationship, and I am not planning on changing it. This post is purely seeking advice for someone else. Namely, my best friend (40F) of several years. We grew up together, and chose to work in the same industry, have been through thick and thin. I care about her a lot, and I am doubtful whether her nonmonogamous arrangement is... well, normal.

Her husband (40M) has always been nice to me and my gf, and I've had a very high opinion of him until recently. My friend and him have been married now for about 10 years, and as far as I know, they have been monogamous, until recently. He was the one who brought up experimentation with nonmonogamy, my friend agreed.

I am currently in another country, and received a call from my gf last night that my friend had called her, asking to stay over. My girlfriend agreed, and apparently my friend showed up in tears, expressing that she might be separating from her husband.

Apparently their arrangement had been that he's allowed to date his coworker that he expressed interest for (and no one else), meanwhile my friend was allowed to date whoever that her husband approves of first. She had a few crushes, but he ended up vetoing them as one was incredibly rich, and made him feel bad, while the other one was an alleged "love bomber". She had another date planned for yesterday evening, and her husband vetoed this one at last minute as well because he was too young for her. They got into an argument, after which she called my girlfriend.

I am not really educated on nonmonogamy on any of its forms. Am I going crazy or does this feel like he just wanted to sleep with his coworker while she stays home and does nothing? I want to be a good friend and not come off as judgmental either. How would I talk to her about this? Any advice is welcomed.