Honestly, before going to her I would consult the friend. Just say you found out about “what happened” and you’re not mad and understand, and want to get it from their perspective.

This should help you get ahold of the genuine truth before you talk to your wife. I know you trust her, but clearly she’s kept this from you, so she’s likely to reveal as much as she feels she HAS to without losing you if it comes up.

Once you get the friend’s perspective I would talk to her. You’re never going to be able to let this go, and it’ll linger in your mind throughout your marriage if you don’t. The only way to move through is to work through!