Individual Saying I owe Her money

My 13 year old daughter was invited on a family vacaiton with a friend. I was told I only had to pay her airfare, which I did. Upon arriving home my daughter gave me a "bill" from the friends mother for lodging and activities totaling over $600.00. The mother claims we "agreed" to this, however, there was never a verbal or written agreement at any time. The mother is sending me emails at work threating to take me to court and charge me interest and for her meals. I have told her I have no intentions of paying this money and have asked her not to contact me at work, however, she continues to send me emails. What can I do to stop what I consider harrassment.

You can refuse to pay and file a police report for harassment if she will not stop--she needs to sue in small claims if she has some issue.