Battery Dies

My mom purchased this car new and has never had a problem. She is alone and lives a few hours away from me. She drives the car about 2-3 times a week. She called me and the battery had died sitting in the garage I just replaced the battery as it was six years old. It has been 5 days and she the same thing has happened in garage with dead battery. Can you give me the most common issues to check out.

Thanks - Walter