Hi
IMO leave well alone.
I did a 'google' search when confronted with the same issue with a glass conservatory roof, and the answer was to replace the glass as it would be the cheapest solution. However, the flashing would need to be changed as well to prevent the problem from reoccuring.
Just tell you customer that you can clean the roof and it will cost X, but he will be disappointed if he expects the lead residue to come off. It won't. I got the job quoting like this. (I also tried to clean a corner of the roof with a tough sourcing pad and grinding paste as an experiment, but it didn't shift much.)
Spruce