What you describe could be anything.
There are plenty of sources for mold outside - such as lawn furniture, canopies etc. This is because the material doesn't dry out properly after getting wet or moist - instead mold grows.
Just because you see it outside, doesn't necessarily mean that it will be inside. But it doesn't harm to have a throrough check around in all nooks and cranies.
Have a look at the site, and investigate some more. Does this friend of yours have any other health complaints? Are they on any pharma medications?