More Useful Stuff
We? No, no. Don’t you and her do anything, least of all sit him down and smack him with a mental image he’ll never forget. The more you conspire with her – or the longer you hide this past fling – the more he’ll think it was bigger than it was.
So grab a beer with him and tell it straight: you two fooled around, it went nowhere, but you think she’s great and that she’s way, way better off with him. Stress it: way better.