I never heard of treating a cedar roof with anything, cedar roof have been around for a long time, never did they treat them years ago, nor should it need to be done now. Cedar is naturally resistant to rot, with a decent pitch (7/12 or greater) they shouldn't hold much moisture. I'm not saying that it can't be treated, I've just never heard of it! If they used a product like Thompsons water seal in a sprayer, it would take no time at all to apply, and it depends on how dry the shingles were as to wether or not you would feel a residue. Wish you the best. Next time try to check into your endevors before you have them carried out, this way you will have no questions after the fact. Not trying to be sarcastic, just helpfull.