So last night when DH got home, he added enough bleach to get it up to 19.5 ppm (the 1.42 gallon jug) at 12:30 (just after midnight). This morning the chlorine level tested at 13 ppm at 9 a.m.
We'll see what we can do about the relative. The problem is that they are Matt's parents and they are joining us midway during our vacation.
What would be the hazards of superchlorinating while we are away (like going up to a higher than normal shock level, or adding a bunch of bleach, and then having them add a bunch more on their way out of town)? Would that hurt the liner or anything?

His parents are on the older side and don't live super-close to us either.