How might you react in the situation depicted below?

 
  Hypothetically, you've lived in your neighborhood for 10 years or more and you're on polite speaking terms with a married couple two or three doors from your place. They already lived in the neighborhood when you arrived. Other than being on a first-name basis and saying hello to each other on passing, you only occasionally socialize with them both or visit each other's houses.  You are closer to the spouse who is of your gender, and the two of you often make small talk. 
  One day about three months ago, you happened to notice that one of them, either the husband or the wife, receives a visitor of the opposite gender without the presence of the other spouse. Just seeing it in passing means nothing, but over the next few weeks, it continues to occur: the exact same visitor stops by about once every week, and never when the other spouse is home. Now that it's become more noticeable to you, it catches your eye even more every time it happens. 
  Gradually, the visits increase in frequency, to twice or twice a week. You have seen the other spouse leave for work and the visitor show up within the next thirty minutes. The visitor's car is usually parked at least half a block from their house, never in front of it. There is even a time period where the other spouse is out of town for a few days, during which time the visitor's car is parked on the street directly outside their house until very late into one evening, and two days later, it's parked there when you retire for the night and it's still in the same spot when you leave for work the next day.
  Last straw: a day ago, as you're arrived home, you see the visitor and your neighbor just outside the house hidden away in a secluded corner where they don't appear to suspect they can be seen. They are in a deep embrace and romantically kissing each other. You only see them doing this for a brief few seconds before they stop kissing, break the hug and look around nervously.  A moment later as you exit your car in your driveway, you see the visitor scurry from the house and drive away.

  DILEMMA: the neighbor with whom you share small talk approaches you later and with a depressed, downtrodden expression, asks you, "Listen, have you noticed anything suspicious going on around my house over the past couple of weeks when I'm not home?"

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