Employee needs to fill in for another: How much can we demand?

Q: We have an employee who was hired as a part-timer, and she has some medical issues. Recently, she was asked to fill in for another employee because of maternity leave, and was told that she must work the other employee’s days whether she was part-time or not. Neither her medical issues nor her part-time status were taken into consideration. Can she be forced to work? Does she fall under any state or federal laws that will protect her from being terminated? What actually are her rights? – Gary, Ohio