Is it tax fraud?

My sister got married in August 2003. She kept it a secret from everyone. I just found out. In August 2004, he was arrested and has been in prison ever since. They lived together for 9 months after they were married and haven't lived together since but they're not legally separated or divorced either. He's never worked an actual job and has never paid taxes. My sister has and she's been filing as single. She said she didn't think of herself as married. I'm just wondering if this is tax fraud and what kind of consequences this could have for her. Thank you very much...

She can still file a tax return by herself in some cases so it is not necessarily fraud.