I agree with Chi.
Make sure the guy knows before you leave that it's OK.
However you want to do that.
Have a few drinks with him. Then the wife excuses herself to put on something attractive/seductive and on her return she teases and flirts to get him arroused.
Then you say, "Well unfortunately I have a few errands to run for a couple of hours, so while I am out you two stay out of trouble" with a big smiley face....
Then kiss your wife and leave....
Darn, I admit that I wish I was you
I've wanted to try this....maybe someday...