Hope this is in the right place - it involved other people's re
lationships but it is a more general question about what to do when you accidentally find something out that could have serious ramifications. If it's better in the rel
ationship section can someone tell me how to move it? I'm pretty sure the answer is that I can't say anything and should just distance myself from all involved and keep out of the mess that is sure to ensue. Long background to follow. Skip to end for shorter question
B/g:
I have a group of old uni friends who still keep in contact on a fairly regular basis - we meet up several times a year altogether and see each other in smaller groups more often. Most of us are married/coupled up to people who we met since graduating. One of the guys, Peter, was out of contact for quite a few years but in more recent times has caught up with us all. He is getting married in a couple of months to the girl he has been da
ting for several years. We have met her a couple of times but Peter usually meets us without her. At uni Peter dated another of the group - my close friend Vanessa. She is, like most of us, already married. I accidentally discovered recently that Peter and Vanessa are having an affair. I'm horrified but, because I accidentally saw a text which made me suspicious and then overheard something that confirmed things, this is obviously a secret I'm not supposed to know. The whole thing is a mess

- Peter's girlfriend recently had a baby and the wedding is coming up. We are all supposed to be going to the wedding but I really don't think I can attend now. I really want to say something to Vanessa or Peter. Actually, if I were Peter's girlfriend, I'd want me to say something to me so I didn't throw any more of my life away on someone who blatantly doesn't appear to love me

Still I guess it's really not my place - is it? Obviously, I am disgusted by their behaviour and certainly don't want to have them as friends any more but I am still going to see them when the group gets together and if I don't tell them what I know they are going to want to know why I have been distant.
Question:
If you find out a secret that has the potential to cause several people a lot of pain because of someone's behaviour, is there any way you can say anything or do I need to keep schtum and distance myself. If I do distance myself, how can I explain that without telling people what I know?