what should I do after finding out this secret between them?

I’m in a tough spot. My husband and I have been married for 10 years and have 2 kids. I’ve always trusted him completely. Recently, I went to visit my sick mom, and while I was away, my best friend found out her husband cheated on her. She called me in tears, and I told her she could stay at our place. Here’s where things get complicated: I noticed that my husband and my best friend had been messaging each other a lot while I was gone. I ended up snooping on the messages and found out that something happened between them. My husband says I deserve to know, but my best friend wants to keep it a secret. I don’t know what to think or do now. I feel completely blindsided and have no one to talk to. Has anyone been through something like this? How do I handle this situation? #betrayal #relationshipadvice #trustissues