You have 8 yards of fabric, which is 8*36*42 =12,096 square inches. (I'm assuming you can get 42 inches in width, which is optimistic.) A very large king might be 120 x 120 inches, which is 14,400 square inches. Even if you lost nothing due to seam allowances and cutting, you would not have enough for a quilt that size. I would go back and explain the math to them.