Oakton Meter reading in PPT (well water)

I have an Oakton combo meter Pttester 35 and I tested a buddies well water the other day and it read 1.45 PPT (parts per trillion) which is saying the water has almost no salts in it. I have tested my city water and well water and readings are normal which read 220 ppm and 350 ppm along with other water supplies so I am pretty sure the meter is working properly. It switches over to ppt when reading his well water automatically. I have seen white residue on his plants which show signs of hard water so I find it hard to believe there is no salts in his water.

What could be causing the reading on his well water to be showing such a low reading? Could it be his water is really bad and its throwing the meter off?
Is there any test I could run to see if its accurate like adding nutrients to it to see if it brings it up to the proper ppm?

Take a sample of his water in a clear cup. Set it aside and allow it to evaporate, then see whats left behind. Or you could it tested at your local health department of ag extension office...the latter will tell him exactly what he is working with.

I believe that will be the only way to find out is to have it tested. He said years ago that he was told his water was really hard with term liquid rock used and he has a fish tank that has a white film that will not come off. Got me wondering if his water causing a erroneous reading. He uses it on his dirt plants with no ill affect but like I said I noticed the white buildup on the leaves.