If you can see the leak - possibly around the vents where those nasty things tend to age and need replaced. Tell him it needs to be done. If you can see the leak inside, what is the source? If the decking is rotting it will look black... Needs fixed regardless. Use a cost to cure estimate from a book like Craftsman's Insurance and repair manual. If it is around the fireplace, it may not be impacting the roof at all but bypassing it. I have one that is a nightmare and an annual inspection and pasting is in order on the top of my fireplace. The water runs down the inside, where it hits the wood ceiling which sets on a lip in the brick of the FP, and then drips out about 3' in front of the fireplace. It is not good for the sheetrock above, and a real "fix" would mean taking the top off the fireplace (its' 4' x 8') and replacing the metal place, waterproofing it, and covering it with waterproof mortar as best possible.
