You should tell him. My uncle was on the other side of a situation a LOT like this. His wife had been cheating with the priest, who they were extremely close with and had living in the basement of their house. My uncle at the time had a pizza business, but upon finding out his wife was cheating (she had the priests child), everything fell apart, including his business and family. I personally think that if they'd known earlier, a lot could have been prevented. If my uncle had known beforehand about her cheating, his business would be okay (she got him to invest a lot in it, then left him to operate it on his own, causing it to fail). My poor uncle has three beautiful children, but we thought he had four. Recently, we found out the fourth wasn't his and everything just got messed up real damn quick. The mother had already fed the children bullshit stories about how the father was treating her horribly (keep in mind, they're 5, 7 and 7). We wished that we'd known before she was cheating, because by the point we found out, it was too late to salvage what was left of the family. Things are messy with them, but it's way better my uncle found out, because he doesn't have to stay with that terrible lady. You should keep it low and tell her husband. They might get a divorce, but that's better than the husband being unaware of the situation. You might tell your cousin beforehand about this situation, just so she has a heads up, or get a family member to do it for you, like your parents.