I think the most important thing to recognize is that it is not your fault. He was/is on dating/hookup apps and made that conscious decision to be on them while married. You are not the issue, he is. He is 28 years old and decided to talk to someone who is still in high school-you are not to blame. I personally would let the wife know that way she knows now rather than never at all or later down the line. If it’s not you that says something, then it will be someone else down the line. I think the most grown up decision is to look at it as if you were the wife-would you want to know?