should be fine. most test kits give you a measure of the amount (n) of say phosphates contained in the water and not the concentration © of that chemical. so if your LFS dude is on to it and has some distilled water lying around, he can add it to the test tube with your water, so a colour change is visible.
the concentration of what ever is in the water will change on addition of distilled water, but the amount (n) will not.
having said that, he is only going to be testing about 4 - 5 mL in a t tube, so it should be fine.
(don't go out of your way to buy those kits)