I trust my dh.
I have never been given any reson to suspect he is not faithful in 17 yrs of marriage.
If he went, as a foursome, to a single womans house, and the other 2 were also single, I would question my marriage.
End of.
There has been lots of speculation about the state of this blokes marriage, whether he has dc etc. We know virtually nothing, including whether or not he is a serial shagger.
I will say again, it is naive to get mixed up in this scenario. You purists may not like it, but society as it is puts sexual connotations on late-night invites back for coffee. That is the way it is.