How to answer question on background investigation form

My wife was written up for two performance issues a couple of years ago. She found new work before any action was taken and when she went to resign, was given a severance package (she had been with the company six years). She has been offered a job with a Federal Contractor and is filling out the background paperwork. She must answer the question if she resigned in lieu of termination, was terminated or left on mutually agreed upon terms due to performance issues. She is concerned that the person doing the background check will talk to her direct supervisor rather than the HR department. Technically, she found other employment, but we do not know what would have occurred had she not found employment. What is the best course of action for answering the background question?