Why would you think it would pose a problem? How long ago did they run the area? Your soil in that area should be great. They don't plan on eating on the ground like a picnic do they?
If anything scoop it out and take it home and put it in your beds.
Tell if I'm wrong.

When did animal raising activities end? This type of animal waste breaks down quickly into fabulous organic soil. If it were my job I would excavate 6" of soil from the spots where decks will be constructed, backfill with poor soil or gravel, and add this manna from heaven to the gardening areas. By the time all is finished and mulched there should be no danger to customers. I suspect the soil test will indicate a rich soil. If there was a board fence around the plot which will remain I would suggest power washing it, and the exterior of the building, with a clorox/detergent mix before painting or using.

Are you kidding??? Gardening on former poultry and rabbit free range will save you $$$ in fertilizers and compost to use after it decomposes. I have used compost from my chickens, ducks, geese and turkey to enrich the gardens of some of my clients. Heck, I let my birds poop right in my own garden. :)

I dump bedding from the rabbits right onto the garden, and have a compost pile of poultry poop and bedding. Don't change a thing, and tell your client to sit back and watch the plants flourish.