You have removed the warning letter from her file and have told Joan that you hope she will keep up the good record of improvement that she has started. Joan may, or may not, be absent on future Fridays – if she is, then that is another resurfacing of the same problem.
Did you ever wonder what Joan’s problem was? Other decision paths in this ‘learning game’ would have led her to tell you. Perhaps you would like to hear what it was?