When I first began reading this thread I just imagined the man involved was young. I understood all of the angst and the concern.
Then it was revealed that he had been married for a long time and had had only three sex partners in that time: the wife, the one night stand, and the current girl friend.
The current girl friend should just naturally assume that a healthy man in his mid to late thirties would have multiple sex partners in life. The knowledge of the one night stand and possible pregnancy should of course be discussed with her at an appropriate time in the relationship.
He definitely needs to be tested for STDs.
From what has been shared about this man, I am not sure he is the best boyfriend material for his current girlfriend. If SHE were my friend, I would tell her to run the other way. Now. He is not mature or healthy enough to sustain a solid relationship at this time if he continues to not be forthcoming to this current relationship.