Their house is single level, but with a finished bonus area downstairs. Plumbing all seems to work great, but when it rains, and only when it rains, there is leakage coming from the ceiling in the basement/bonus-room downstairs.
From where the leaks come through, it appears to be just below possibly the dishwasher upstairs or the bathroom.
Funny thing is that it happens only when raining ! And, there is no sign of any water running down the pipes, etc in the attic or on the main floor – just from the ceiling in the downstairs in the finished basement.
Of course a great plumber could come in and tear out the walls looking for the problem, but these nice people are terrified of having to pay $$$$$ to solve this small problem. They are retired, and want to know the best most efficient way of diagnosing and fixing this.
My best guess would be that this leak comes from the roof via the venting pipes that serve the upstairs bathroom. Your post implies that the leak is unrelated to the use of plumbing fixtures, and occurs only when it rains, whether or not fixtures are being used. That indicates that the water you see is rainwater, most likely tracking from the roof to the vent pipes.