Neighbor's hot water heater burst and flooded my condo. His insurance wont pay. (California)

I own a condo that shares a common wall with another unit (owned by someone else). The other unit had a hot water heater that burst and flooded my unit.which cost me approx $4000 to dry out/repair. I filed a claim with the neighbor's insurance and was declined because "it was a sudden, unexpected event of which our member had no prior notice of a condition. Although unfortunate, it is not the result of negligent activity on the part of our member. Therefore our insured is not legally liable for any loss you incurred." I appealed and asked to see the documentation and evidence they relied upon to make this determination. I received a response that they are under no obligation to share their files with me and the burden of proof is on ME to show their client's negligence resulted in the damage to my unit. Do I have any recourse? I feel that I am the innocent party... and would like to be made whole. The condo assoc insurance also declined me because they only cover damage done by things inside the walls, on the roof etc and this damage was caused by the water heater inside another unit.