Tim,
If it has been on there for more then a month or so, I dont think you're going to get it off. Especially if the house is not vinyl.
Dont make any promises, because you will most likely not be able to keep them. There are no products that I know of that will effectively remove shotgun fungus. If it is aluminum, don't beat on it too hard........could do more harm then good.
I had heard that a company in Ohio was doing research on this.......but I have no idea what their progress has been.
So, tell the customer you may be able to remove some of it, but not all of it. The sooner you get to it, the better your chances. Also, instruct them how to prevent it from happening in the future..............use inorganic mulch, replace mulch every year, etc.
Do a search on this.......probably more info........also search "artillery" ........because it is sometimes called artillery fungus.
Good luck.