I have an employee janitor who has girlfriend do 50% of his work. He gets 100% of paycheck. She's ok with this; doesn't want another paycheck job. They live together & have joint checking. Is there a legal problem here?

I have an employee janitor who has girlfriend do 50% of his work. He gets 100% of paycheck. She's ok with this; doesn't want another paycheck job. They live together & have joint checking. Is there a legal problem here?