JRuss, that is potentially very upsetting info. But I really think it depends on the particulars. I could imagine something like that taking place w/ your wife and best friend having the best intentions. So I encourage you to get more info before going too far in one direction.
Are you suspicious that there may have been some romantic connection between the two?
Do you feel like details of the marriage that were irrelevant and private were shared?
Were there issues that your wife really could not talk to you about first, before she went to your best friend?
I'm somewhat sympathetic to this situation because I'm guilty of having talked to close friends & family about my situation. Some of it was just me needing to share my frustrations, and ... well, in my case, my W simply refuses to discuss anything w/ me so it's not like I can talk to her about it first.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final