Legal Question in Real Estate Law in California

The HOA, unless you have done something that altered the the way water gets to, collects on, or flows from your balcony.

You and your neighbor should try to get the problem corrected ASAP so that serious, expensive damage can be avoided. The problems should be reported to the HOA if it hasn't been already, so it can take steps it thinks are necessary. You need to get a runaway train stopped before arguing about whether the runaway was the fault of the engineer, the conductor, or a trespasser.