I believe my buddy (25M) is planning to cheat on his fiancée/wife to be (25F).

I (25M) am wondering what I should do about this situation. One of my closest friends (25M) is planning to get married with his fiancée (25F) in a couple of months. In fact, they are still currently building a house together. To make things more complicated, my fiancée (24F) is the bride’s best friend. My bride and groom are from the same town and have been together for 8 years. <br><br>This summer we went to an after-rave party in a large city about 2 hours away. At this party there were several people from my friend group and some people we didn’t know. I noticed this one girl (23F) who seemed to be alone. She gave my friend a lot of eye contact and shortly after I decided to go to sleep, they were suddenly talking together. I didn’t really find it odd since we are all friends and was also tired from partying. <br><br>I woke up a couple of hours later and they were still talking. Actually, they were both really drunk and making out with each other. I decided to wake up my awake friends and inform them about the situation. We didn’t want to ruin the mood of the party, so we just said that we are going to a hotel and drove my friend with us. <br><br>The party went on for another 5 hours, so I assume they did some stuff in the car. We were pretty much silent about the subject with my friend. We haven’t spoken about it, our friend group or my fiancée, since then. This was three months ago and I haven’t noticed anything bad from my friend’s behavior. <br><br>Last week I was partying with my friend again and we went to his place by taxi. He was trying to get the girl’s (23F) number from my phone. He was still a bit drunk and shortly after he fell asleep. He still didn’t wake up when I left the house, so I decided that it is better to delete the number. Today I received a text message from the girl and she was wondering why she can’t contact my friend anymore. I said that he gave me the wrong number and want to see him later this week. She said that she bought some lingerie for their next encounter and I said that we are probably going to an after-party. The girl asked if she can come with us and will buy me a drink if I arrange it. I said yes, since I still didn’t had the courage to tell the truth. <br><br>I want to tell her, but I don’t know how bad it will ruin the relationships in my friend group. Both my future wife and the bride are really good friends, and I don’t want to ruin their friendship. Does anyone have any suggestions about the issue? I want to be honest to everyone and not participate in their cheating. <br><br>TLDR: My friend is trying to cheat on his fiancée and I am acting as an intermediary.