John owns two lots and Boulder, Colorado. These lots have a beautiful view and are undeveloped.

John owns two lots and Boulder, Colorado. These lots have a beautiful view and are undeveloped. Neighboring landowner, Mary, sometimes uses a part of the lot when she has outdoor parties for her guests. When this happens she tends to use part of John's property on the nearest lot. This goes on for about 20 years. The parties are the only time Mary uses the lot. During the first year of use, Mary asked John if she could use a small part of the property for a guest party, and John said that would be fine, but to remove all tables, etc, after it was over. Mary did not get permission for the next 19 years. 

John decides to build houses on his two lots, but Mary files an action claiming that she is entitled to the part of the lot she used under the doctrine of "adverse possession." Who wins, John or Mary, and explain in detail why?