@ineke don't stress too much. All is not lost.
The store person made some rookie mistakes, like using an opened bottle of calibration fluid to test. Which could be contaminated or expired. Calibration fluids have an expiry date.
All he needs to do is get hold of some new 1,413 µS/cm conductivity solution and recalibrate the TDS pen with this calibration fluid.
1,413 µS/cm conductivity solution should give you 342ppm TDS reading.
I don't see why he would need to get specific Hanna calibration solutions. I checked the website and Hanna sells calibration solution that is prepared against a Standards based formula. So any similar certified solution will be enough, since it is a known quantity that the TDS pen is measured against.