Nitrate levels may be low, but have you checked what the phosphate levels are? Pictured below is an example of what good water quality analysis will show up. The nitrate level (right) is a low 10mg/L, but notice the phosphate level (left) falls between 5-10mg/L (using the highest dilution factor)! Phosphate levels should be maintained below 1mg/L to help reduce algal problems. This is one example where no matter how much chemicals you keep adding to the pond, it won’t have a long term effect.
So where to from here? I would advise the owner to perform several water changes immediately. Then, check what type of foods they are feeding them. Poor quality foods tend to foul the water in this way because the nutrients in the are not as easily assimilated by the fish (see previous post). Providing shade and using natural barley extract can also help. If reaching for chemicals, simazine is one that’s safe to use with fish. More thorough look at algae and treatments can be found in a previous post – see link.
