Job Interview Dilemma - Please help?

I am currently working for an insurance company (with an independent agent) and I am interviewing for a position within the company on a corporate level. I am scheduled for an interview next week and I am not sure if I should tell my boss or make up a story on why I need to be out. Another factor is that the person who confirmed my interview is our client - therefore he knows me and my boss. I am afraid he might have already tried to contact her. What should I do? Should I tell her the truth or make up a story on why I need to be out. I am afraid that if I lie to her - she will fire me. And I am also afraid that if I tell her the truth - she will get mad and fire me. Thanks for the advice.