On a hot day the roof temperatures can get hot enough so that the granules can move when you walk on the roof, the hot water from a pressure washer can easily get much hotter.
I would show customers the brochure of the surface cleaner, how hot it can handle water, how many gallons per minute and how much pressure and show them that it takes at least a couple thousand psi for the swivel to turn the spray bar to clean decently, more pressure the better for blasting dirt and mold out of the concrete, people just love to blast away on concrete all around the country, just blast, blast, blast away and this guy is doing this on your roof where the granules are not secured that good and the hot water loosens the asphalt so they come off quicker, this is a wonderful way to remove the granules that reflect the heat of the sun away from your roof.
Remind the cusomer that most of them use bleach for their white clothes to get them clean, it is also a great mildewcide, disinfectant, cleaner, etc.....that most people have been using for generations with no problems, restaurants use it to sanitize their dishes, utensils, pots, pans, etc....
If the bleach was really so bad then they would not let people buy it at convenience stores, grocery stores, corner stores, shopping clubs, etc.....
Fear Tactics is really sad but the reality is that some people will listen to him and let him destroy their roof. You can make a brochure or post card to send out explaining the truth and expose the fear tactics so that people understand what is really happening or put the info on your website for them to read.
It might turn into a game of who is the better salesman and the winner gets the customer and the job.
Good luck.