Unlike my previous post this is my genuine position: I think it’s entirely up to Crime Scene to decide whether to tell the wife and we don’t have enough information to say say whether he should one way or the other. It completely depends upon their relationship.
In a super-romanticised, lovey-dovey, no-secrets kind of “ideal marriage” then yes he should tell her.
Most relationships though, and frankly most marriages, are not like that. And as it almost certainly, practically speaking, will hurt her a whole lot to know this, it’s very much up to C.S. to judge where the lofty principles intersect with the real situation “on the ground”. Admittedly, I have not had a proper successful what I would define as long term (>2 years) relationship, so feel free to discount my advice on that basis, but it is still from observation of others’ and also shorter ones (which presumably don’t count as successful but not for this reason).
Other things to take into account include whether she will be the last to know - presumably not because others in this group of friends are going to eventually notice you ignoring this guy, and you will have to come up with a reason. If you or your wife are going to tell anyone else about this then I really think you need to give him a right of reply first. Similarly, if you pull out of his wedding you have to confront him about this and give him a right of reply.
The facts so far are, for example, consistent with the knickers being washed with the bed clothes, him going upstairs for a cheeky wank, him finding the knickers and chucking them on the floor, and you coming in at just the wrong time. They are also consistent with him just picking up any random thing from the laundry to wipe himself off. I am not saying either of these situations are plausible but they are possible. That would only be fair.
And if you don’t pull out of the wedding you are going to have to let this go. Also, if you pull out of the wedding his fiance may find out.
One thing though - if you do ruin his social life and marriage, which is potentially a worst case scenario, it is still ultimately his fault, if that makes you feel better.
*A brief digression on it harming her - note that I don’t want to say that this isn’t my opinion or is my opinion - it’s how I see things in a general sense but is not a reflection on this incident:
In a completely materialistic sense her being upset would not be a rational reaction, and you can see that even better when you consider the nature of the reaction. For example she will almost certainly want to way to throw the underwear away. This is objectively completely irrational - you can always clean them (and they were going to be cleaned anyway so it’s not even extra washing).
The reason she would want to throw them away is because of [sacred contagion](Sacred contagion).
And it’s basically human brain misfiring that causes us to over value abstract principles in the first place, which is what trust (in this context, I am not talking about trust in general) is.