I agree with Watermom on the stabilizer, but other than cleaning your cartridges if he added it through the skimmer, there's not much you can do about it now except avoid trichlor/dichlor.

The chlorine block you have doesn't test for combined chlorine, only total chlorine. And although it only measures up to 5 ppm, you can always use CarlD's shotglass method to expand the range--use one part (shotglass) pool water with one part distilled water, mix thoroughly, test your sample from that, read the results, and multiply by 2. You lose a little accuracy with the dilution, but it gives you a good ballpark.

And if he doesn't test for calcium with your pool/equipment, I sure wouldn't use them for my pool maintenance!!!!

Janet