I would write a letter to the woman formally asking her not to feed your cat as he has a medical condition. Get a tag for his collar such as this diabetes one ( my cat HAS got diabetes so wears one )
Cat Tags
In the letter tell her in no uncertain terms that if she continues to feed the cat, knowing that he has a medical condition, you will report her to the RSPCA who WILL deem this to be animal cruelty. Oh, and you will sue her for the vet fees.
She might not look so smug then.