My wife, now XW, had a special friend (the OM). We were at Disney World when I first discovered they'd been texting each other all day long even though we were on vacation with my sons. I hit the roof; I actually told her I wanted a divorce while we were walking up the ramp to the monorail. I was one livid SOB.
That night, after the boys went to bed, my wife and I talked for several hours. She convinced me that I was just jealous and that her friend was just a friend. I bought into the nonsense; I was an idiot.
We eventually went to an MC and the MC became my IC. This is what the IC said to me, "Listen, this is how marriage works; if one spouse doesn't like the other spouse's friend, then the friend has to go."
That's a quote worth memorizing. Don't let your wife sell you on the friend nonsense. The friend needs to go.