rental application fee

[Ping W.]

An applicant visited our house and showed great interest in the house and submitted application onsite but on my way home. I received a text message from her saying they didn't think they would pass the screening process and didn't want to waste their money. They would like to withdraw the application and asked me to mail their application fee back. On the application form, it says, the application fee is non refundable but I have not started the processing process yet. Should I refund her or do I have to refund her? I spent 3 hours with showing the house to her. 

Any input will be appreciated!