Hi, my name is XXXXX XXXXX I have been in the plumbing and plumbing repair field for 20 years. I will be glad to help you today. The damp area is inside his house?
Very unlikely. What investigation has brought him to this conclusion? How was he able to exclude anything from his house that directed his attention to yours?
Funny how there was no issue until the new installation. Could be ground water or very likely his own plumbing. Your plumbing does not connect to his.