Has your managing agent and super inspected to make sure his over flow is open for water to drain out?If not he needs one. I would contact my insurance company and explain the problem they will bill the upstairs neighbors insurance company for all work that is needed. Example mold removal, plastering and painting. In the mean time the neighbor above needs to be aware of this problem and also have him contact his insurance company. You as a board member and president you need a managing agent (a good one) who can advise you on problems that is going on in your co-op. Let us know the results. Best of Luck and Happy Holidays.

I had a water leak from my upstairs neighbor, who left his toilet overflowing due to dementia related problems. I contacted my insurance company who brought in equipment to dry out the
apartment, remove damaged wallpaper from bathroom, and replaster and paint,
. The insurance company paid for our hotel while the work was done, and the cost of damaged items and restaurant meals. Of course, we notified the Management company. They sent a building insurance agent to inspect.
The upstairs neighbor was a renter, unlike us. I don’t know what his liability was. He soon afterwards moved to a nursing home.

Thank you peoples choice #1 and H for your responses. We are a self-managed building. The Board had a meeting with the upstairs neighbor last night who is very contrite and accepts responsibility for any costs incurred by the downstairs neighbor. Both have insurance. The upstairs neighbor agreed to buy Flood Water and Overflow Alarms (and did so immediately after the meeting). We also encouraged him to turn his phone off before turning on any water, which he agreed to do. This we believe is a distraction. He is on medication and that may be affecting his behavior. We told the downstairs neighbor that we will be having a mold inspection of the entire building early in 2019. It's necessary because of this flooding and it is also now required by the City.