do you mean composition shingles or cedar shake shingles or tile shingles?
If I were in your shoes, I would have the siding inspected and if it is asbestos then make sure the homeowners take care of it and I mean remove it. Painting over it is only a bandaid and you will have to fight this battle only again. Get it fixed right and when you plan to sell this home, you will not have to fight this once again battle yourself.
If they do not want to spend the money to have it removed then, take a step back and re group.