Work Resignation

Can you be fired after you gave them your resignation letter? My husband handed in a resignation letter last week. He gave a two weeks notice. He wasn’t able to come in for work that day because he’s not feeling well but gave the letter anyway. He promised a doctor’s note but it would be this week since we had to schedule a check up appointment. He got a doctor’s note yesterday and handed it in when he went to work last night. Today, his boss texted him that the doctor’s note can’t be accepted since he is already fired. I just want to ask if what will show in his employment records. Is it fired or quit? Thanks!