I know what you mean ... I fully understand it.
But an elderly lady may not be able to have a pet. And she's probably on a limited income (who isn't these days?). And she still found the funds to buy a bag of dog treats for strangers ... to be nice, to make contact.
I would thank her for her kindness and thoughts, but advise her that my dogs have food allergies to so many grains that I can't accept her offer. And then thank her once again for her kind offer ...
So it's a lie! Mine aren't "allergic" to anything (so far!!!). But they're finicky about their treats (they're a tad bit spoiled
), so I could get away with it.
Some people deserve to have their feelings hurt/ignored; others, well ... not so much.
Just my opinion and how I would handle it ...