Possible MHP Backflow Issue?

[Perri Weber]

I am selling a 46 space MHP that was built in the early 60's. The park is on city water/city sewer and the water pressure is low, but in the range of normal. The buyer had the water tested twice. They are saying that when the water is in back flow it is below the requirements, which could be a potential health risk. The estimated cost associated with this is $100-$150k (from the buyer and seems high). The owners have never had a complaint about low pressure and have never experienced a problem with back flow. 

Has anyone had any experience with backflow issue in MHP? What was done to prevent it or fix the issue? How much did it cost?