How do you think he feels in the first place, and now that you've posted his mistake all over the web? It's one thing to post about our own mistakes, but to rant over someone else's, particularly someone like a brother-in-law, is certainly lacking in consideration.
OldPhotos "If everyone possesses some measure of this intangible quality called creativity, photography is unprecedented as an outlet for its expression." - Ansel Adams
A yellow duster is a yellow coloured cloth sold for househould cleaning and dusting. They are generally labelled as lint free and after a short time of cleaing they generally are, mainly be dropping yellow coloured lint all over the surfaces you were trying to clean. More normally they are used for cleaning brass, silver and other metals in conjunction with brassoor for buffing drill boots after polishing with kiwi parade gloss (note: Kiwi brand shoe polish not a kiwi which is a flightless bird from new zealand, nor a kiwi which is a green tropical fruit).
The use of a yellow duster to clean a camera is proof enough of incompetancy since it tends to add more dust than it removes.