Can my boss fill out my time sheet?

My friend and I are wondering if this is legal. He was out because he was sick a day and his boss filled out his time sheet when he was not there to turn in. now they were not due yet and his boss just wanted to get them in so she filled it out and turned it in. he returned to work the next day and filled a time sheet out but when he went to turn it in she explained she had already filled it out. we are wondering if this is legal. Anyone with helpful knowledge about the subject please let us know.