Hartz is the brand that kills. I don't think they're all toxic, but anything by Hartz is a death sentence to pets. Can't do anything for someone else's cat, but will eventually check to see if it is a Hartz collar and find the owner and talk to them if it is.

New neighbor kitten just spotted, wearing a flea collar. A little skittish at first, but eventually came up to me cautiously and sniffed my finger.