Sweating Moldy Ac Vents

This article will explain why AC vents sweat and potential causes LINK.

Need to get into the attic and check to see if the “boots” - where the ducts tie in to the register - are properly sealed (not leaking cold air) and properly insulated. That is often the most common cause. But also insulation on the entire length of the ducts in general - assuming they are rigid ducts, and not flex ducts.

How cold does your friend keep his house - the colder the house the more likely a problem like this can crop up. Also likely need to check-monitor the relative humidity of the interior of the house. All this is discussed in the attached article.