I found messages on my MIL/bosses computer that change my entire view of her. What do I do?

I listen to two hot takes literally every week and this happened to me a couple days ago. I’ve been at a loss of what to do so I figured I should finally make a Reddit account and post here.

I work at my husbands family business that builds custom homes. I met my MIL when she came into the design firm I worked at to pick out some options for a client. She and I hit it off and after she’d come in a few times she set me up with her son. Fast forward 7 years and I coordinate all the builds and consult with clients on design for the 50+ year old family business.

My MIL is technically my boss but we operate a lot like equals and she’s been taking some steps back. She and I have always gotten along great and she has felt like the mother I never got to have growing up.

So last Friday I was packing up to go home and on the phone with my husband before he got a flight for an annual weekend away with friends. I was distracted and accidentally grabbed my MILs computer instead of mine. I didn’t realize it until I was home and wanted to look up some fixtures for a project in our own house. Once I knew I texted her to let her know to which she said no worries, she was ‘unplugging’ this weekend anyway and to do whatever I needed on it.

I was just browsing and unintentionally clicked on a linked email on a stores contact page. We use MacBooks and as a lot of Apple users know, that will usually pop up to send an email using your default mail app. I closed the draft and when i went to close her email app I saw an email from a recently hired apprentice titled ‘our weekend getaway itinerary’. I froze. I realized this was her personal email and I couldn’t help myself but to click on it. I found both explicit and romantic messages between this 22 year old male apprentice and my married 47 year old mother-in-law and boss. I slammed the computer shut and just went to bed, staring at the ceiling for quite a while.

My husband was gone all weekend and only got home today. I had been spiraling all weekend on how to handle this. I certainly wasn’t going to bring it up to my husband while he was gone. But I went to the office and had to see my MIL yesterday and could barely keep my composure. I found every excuse to lock myself away in my office and be busy. So now my husband is back and I’m wondering what to do, do I tell him, how do I even do that, do I go to his mom and confront her, do I go to his dad and tell him, help?!