Another condensation question

My husband and I own a steel roofing company and about October of 2005 we installed a Decra steel roof on a customers house. For the 4 years he has had his roof it leaks every winter, however we have had many a spring and summer since his installation that New England has had torrential down pours and they have never experienced a leak during those periods of rain. We used a 30lb Tri-Flex over the entire roof system before installing the steel. Also, ever since we put his roof on he atrted going to Florida during the winter months, but comes back occassionaly during the winter. He has told us several times that its when he turns the heat back on in his house that the leaking begins. This past year we suggested that he have his ventilation and insulation checked by proffessionals in that area. Since then he took it upon himself to install gable end vents (he already had a vented ridge) and had a proffessional insulation company redo his insulation. Now he is trying to bill us for all of that plus for the painting he had done in his house due to damage from leaks. I don't feel responsible for his damages, we are a proffessional company who stands behind our work but this is ridiculous. From what Iv'e told you, is there anything we could be at fault for?