@Kate270
Hi Kate, that is definitely a threat and will not be tolerated by Airbnb. If this person is the account holder go to her profile and 'Flag' it!
Under the user profile name you will see a box that says 'Report this User'!
By doing that you will bring it to the attention of Airbnb, you will be presented with a 4 choice box asking why you are reporting this user. State inappropriate contact, quote the booking reference and highlight the threat aspect and Airbnb will take care of it from there.
I am sorry you have had to strike this so early in your hosting career but you are doing a good job. I particularly like the way you respond to every review. Not enough hosts do that, and I feel it is like closing the book at the end of the story. Good on you!
Cheers.....Rob