To know what I would do, I would need to know more about the people involved.
If I had reason to believe the guy was behaving way outside his norms, he had never done it before to me or any other woman to my knowledge, and he was really drunk I would speak privately to him, telling him what he had done, saying “Never do that to me again, drunk or not, or I will knee you in the nuts,” and that he needs to avoid the booze to that level of intoxication.
If the guy was a player and I knew it, I would tell him he sucked, and he had better not do that to me again. I would avoid being with them as a couple. I still probably would not tell her.
The thing about telling the wife is that you have to be prepared to lose the friendship if you do. If she is not ready to leave her husband, she is most likely to kill the messenger. He, if a snake, could easily tell her that you came on to him. Then it’s he said, she said.
The one exception I might make was a lifelong friend that I knew would believe me, and that I knew would want to know no matter what.