We lost our darling cat a few weeks ago to the dreaded C (thread on here at the time), and are temporarily cat-less. However in the mean time we are being used for Dreamies and strokes by next door's cat, who pops in and out of the cat flap quite happily.
He's about 9, 10 months I would guess. Adorable thing. Neighbours haven't had him snipped or registered him with the vet, and judging by his fur, they're not treating him for fleas either.
I know I can't actually get him neutered 
But, can I bung a bit of Frontline on him or is that crossing a line? I don't want to have to lock the cat flap, but nor do I want him bringing fleas in.
Neighbours are generally shit, there's no way they'll treat him even if I buy and donate to them. Nor, alas, will they sell him to me, already asked .