Good! Borax, borax, borax. Dose with Borax, wait 2 or 3 hours, retest, redose. Repeat. That pH needs to be brought up above 7.0 ASAP!!! It is acidic if it is lower than 7.0 and can damage the pool! That is the most critical issue with this pool.

You are a good neighbor to help them with their pool!