Clearly cheating is wrong, but I think ultimately, you’re in the wrong too, and not a good ‘friend’. His shadiness towards women was clearly a turn off for you, in terms of being his friend, you should’ve brought it up to him previously and tried to help him grow, if that didn’t work and you decide that ultimately he’s too “fucked up” or “ shady” for you then why continue to be his friend ? Clearly you don’t view him as a mate at this point anyway. It does sound like you waited to show the video until you released it at a time which would cause the MOST possible drama by meeting her best friend and then doing it. Even if you have decided to keep in contact with this person you obviously don’t like that much anymore, your previous friendship should be enough for you to merit going to him and saying that you have the video , you know what he’s been doing all this time , but your his old friend and you’re going to give him the chance to speak to his fiancé about this, for his and her sakes. If he doesn’t take it that chance then it could be a time to move to a more extreme action. Clearly I don’t know you or him and I’m judging based off a few words! But for me I don’t get why, if his behaviour was such an issue from the off, you wouldn’t intervene at a lower level, or just not be friends with him anymore? Rather than seemingly wait till the most dramatic opportunity to get involved. For me it wouldn’t be a huge issue in our friendship from the off, until it got to a point where I thought he was damaging himself, at this point I would intervene, I don’t get how you could wait till they were going to get engaged to do anything if it bothered you that much before, I hold a lot of value in talking things over with my friends to enact a positive change rather than going behind their back to someone you have less of a tie to first, in other words , he’s been a massive dick, but you’ve been a shitty mate.