Title says it all really, accidentally logged into his emails as my he was signed in to gmail on my laptop and there was a slightly over-familiar message from someone he knows (shared hobby) so I clicked on it. It was suspicious but not clear so I went back and read the later messages and they were very explicit, there's no doubt here.
Not really looking for sympathy but concrete advice about what to do, are there things I should do now before I tell him I know? We work together (me part-time) and have 2 sons in their late teens, other woman is married in similar circumstances, I didn't see any sign of them planning to run off together anytime soon.
He's my best friend and my only serious boyfriend, I don't have anyone in real life I feel I can talk to.