Box of Frogs
Zamoras Left Boot
Is she hounding you?
Or get a rescue dog that looks the same and give her that
Or just tell her you've had it put down.
Not a lawyer, but I studied law at college. This might come under the heading of a Social and Domestic Agreement. Broadly speaking the contract will only be legally enforceable if both parties intended it to be so when they signed it. ('Intention to create legal relations' I think is the phrase). Sometimes you'll see a clause about 'both parties agree to be subject to English Law' or something which may be sufficient to trigger that. If that's not in your contract basically I think you can tell the batty old fruitcake to piss off and there will be nothing she can do.
Alternatively I could be talking out of my arse, but hey, it's my two-pennyworth FWIW
It all comes down to whether or not you have a fido-ciary duty to consider the dog's health.
You could check this out with a Chum or Pal. I'm sure they could give you a Lead.