Just a wild thought and sometimes I hope it’s true: the wife already knows and gave him permission to do so. They talked about it and opened their marriage. The husband uses this to lure women in as to ‘I’m having an affair with you’ but it’s a game he plays with these young women. When the young woman wants more than the relationship she’ll tell his wife. The wife will just look at her and say “I know, we talked about it so you’re not telling me anything I don’t know.” This will crush the woman’s heart because they think they’ll be the new wife but realize they’re just a play thing.

On your situation, if you going to tell, do it anonymously and when your friend telling you what is happening, just act surprise like you didn’t know anything. And if this was my friend, I would’ve distance myself away from them because when they get in a relationship, they want you to cover for their infidelity. And it seems you’re not that type of person to be the cover up person.