It'll just be me, but Dotty I can't see the relevance of your post about the OP's profile. Seems snide, but I'm sure you didn't intend it that way.
Eurox, either put the key in the person's hand, or else post it recorded delivery to her, sit back and expect nothing else. It will cost the owner to sue you (for what? Trespass = damage done -- you were in error but haven't caused any losses). If the owner is daft enough to throw money at a civil lawsuit, well there y'go. But it would be an expensive hobby for them.
On the other hand, if they are being intimidating towards you in this matter, that's illegal and the police would be interested.