What kind of worm? As Panacur doesnt do the tapeworm related to fleas.
I would give a drontal and do them regular, it doesnt sound like the owners are doing much if the cats live and eat at yours
Tablet wormers dont stay in the body like spot ons so safe to use regularly, some kittens are done every 2 weeks if infested
You'll also be pleased to hear that worms can hibernate in muscle tissue and migrate when they wish which is why regular worming is important
You can get drontal online v.cheaply from VetUK, i'd do both of them
Lots of rescues have links to vetuk that earn commission, so dont forget to use one of those!