"she only met him once for coffee and had sex " 😂,

My wife [29/F] and I [28/M] have been together for 4 years and married less than a year. Let me also preface this by saying I have a cold and am in desperate need of sleep.

My wife, Katie, and I, started dating when I was in grad school. We were in the same social circle. She was in a relationship with a guy we’ll like “Mike” and it was clear it wasn’t going to work long-term. Mike was a “free spirit” and had a lot of personal issues, so none of us were surprised when they broke up. He decided to go on a retreat/road trip with his ex-girlfriend for six months and disappeared. She got over the relationship and about a year later we started dating. It was an odd coupling, we’re very ‘different’ – she’s a lot more artistic and free than I am; we’re on different levels in the looks departments and many of the things that make us who we are, are seemingly opposites. I’m more buttoned-up and traditional and she’s not. A couple of months in, we hit a rough patch but we worked through it. From then on, things were great. We moved in, I proposed, we got married and that brings us to today where we’re trying for a kid.

My wife’s frenemy “Danielle” has never really been supportive of our relationship and they’ve been friends since middle school but have had several falling-outs over the years. Recently my wife couldn’t/didn’t attend an event hosted by Danielle, which prompted this recent outburst and along with it the end of their friendship. Danielle sent me an email saying that when Katie and I got together, she was unsure of which person to choose so when Mike returned, she went to him to talk about them. They met a few times during the first two weeks he was back and apparently had sex “once or twice”. According to Danielle, Katie ended it and never told me anything about it. She confided in Danielle about it, years ago, and I guess thought the matter was closed. I got the email this morning at 10 and was surprised Danielle emailed me and read it. Probably 30 minutes later I get texts from my wife asking if I got an email from Danielle, and did I read it. When I answered in the affirmative, she sent me a deluge of long texts explaining that it did happen, they had sex once and had coffee three times and that she hasn’t spoken to him since. I’m not sure what to respond.

I have a cold, and have been trying to get work done so I can rest when I get home. I don’t even know how to deal with this. Part of me wants to go home, get into bed and sleep, pretending I never found out. Another part of me is furious that she hid this from me – I had a feeling she had seen her ex, but she then came to me a week or so later and wanted me to confirm we were exclusive and I wasn’t seeing anyone else. Part of me is terribly mad at Danielle for dragging this shit up years later. I don’t know what to do here.

Is this that bad of a transgression? It feels like it was so far in the past that I don't know how to respond to my wife or Danielle.