What’s missing from this saga is what caused the flooding. If it was water pipes outside the walls – like washing machine hoses – it is probably the owner’s responsibility if it was inside the walls, it’s probably the Owners Corp.
The neighbour has no ‘duty of care’ in this although they could have acted a little more promptly. But I’d say if it’s a common property fault the Owner’s Corp should pick up all the bills but if the fault was with personal property, then it’s the owner’s responsibility, including the cost of the locksmith