I have known people who swear by seresto, but my experience with the collars was not great. They are expensive, and IIRC Amazon sells them but they are not the same as the ones you get from your vet and have caused some serious reactions. Can you use a topical?
I have used Zodiac Spot On in the past for Nobel and Lily. be warned that any cat can have a reaction even to a realitively safe medication. Each cat will have their own sensitivities and potential allergies. You need to discuss use of products with a veterinarian. Always read the warning labels and keep watch for what the label says
It doesn't sound like anything anyone here has suggested will meet your needs or the owner's approval. So, go with what you/they want and hope for the best. As mentioned above, most flea collars are not break-aways so that is an added issue that you must be mindful of. If you do find one that is a breakaway, then better buy a bunch of them. Good luck to Scotty and you.