if your grinding then its mixed with poxy arleady and is fairly safe, though will still mess you up if you dont wear a fabric mask (the descent kind form home depot is what i use, its also rated to prevent inhaling some chemicals in paint, little hard to breath but overall worth it.
if you just cutting fabric, dont worry about it unless its extramly fine (under .5 oz)
All this from one fiberglass job, a small telescope tube.
theres no way, there must have been some other reason with it. its not possible to breath in that much dust making a small tube. (unless he sanded it day in and day out for weeks)
wear a mask but dont kill urself over it.