Can't help gently pointing out that, despite assurances from my colleagues @Daniel Hyman, @Ashish Acharya and @Basit Siddiqi, it is not automatically "100% deductible."

The first thing I'd do is try some gentle cleaner, like a deck cleaner.

Turned out the attic had no ventilation (due to the age of the house) so all the moisture from decades of rain and such covered the attic with mold.

Gently speaking I think we are reaching diminishing returns.Focusing on the problem at hand the group has generated several viable options that her brother can explore.I would say that keeping it civil with the landlord and avoiding legal threats will probably help get to a solution faster assuming the landlord is a decent person.

Does she have family (they might be adversaries but you have to make the call and try remember kindly gently) ?

I always keep a fan in the bathroom pointed as high as I can get it to keep moisture level to a minimum.

Just trying to figure out how the moisture got into the building.

It lasts longer, looks better and does not mess up the windows or allow in moisture to mess up the walls around the windows.

:PBecause we all missed the fact that the inside wasn't finished out, doing what I said above ... either a quick fix or a proper finish out, will help tremendously on any mold or moisture build up issues. 

I would check windows and doors also for moisture to see how well they are holding up too.