hello Aitch and welcome to Purrs. Sorry to hear about your friend's cat. Eighteen sounds like a great age for a cat, but a well-cared for cat can live into its twenties, so there is no reason to assume the worst. Someone here will give you the link to hints on how to find a lost cat. In the mean time, call all the local rescues, the veterinary surgeries, and (on the more morbid side) the local authority who will be able to put you in touch with the department that deals with animals killed in road accidents. Make up lots of posters and place them all around the neighbourhood. Knock on doors and ask people to look in their sheds (if you feel confident enough, ask them to do it while you wait. Cats get themselves locked in the silliest of places, and sometimes end up there for days. Walk around the neighbourhood at dawn and dusk calling. Wait long enough between calls to listen for a response (surprising how many people forget that vital period of silence). and good luck. Fingers crossed for a happy ending.