Lou Lou, I feel for you. remember though that it's a cat and not a child. Did you say it was 17 years old? That must be a good innings for a kitty.
These things do happen, except it's normally in sitcoms. Just be straight down the line with your neighbour, they will have had a good holiday and so should be in a good mood. Get them some flowers or something.
Think on the bright side. One less cat means a few more Robins and Blue tits.