My flat was damaged when the toilet cistern in the flat above me overflowed while the owner was on holiday. His insurers will not pay out, on the basis that the owner had turned off his water supply before leaving and the leak was therefore unforeseeable. I feel that, since there are two full water tanks in his loft, he should have known that a leak was possible even if he turned his water off.
If you have your own insurance and the damage is significant, you should inform your insurers so they can pursue the claim on your behalf. It’s unlikely that your neighbour’s toilet will be supplied by water tanks in the loft. They are usually connected to the mains supply, which suggests he didn’t turn his water off before going on holiday. If you can establish this, the insurers may change their tune.