Is it legal to fire an employee that refuses to work overtime?

In terms of employment, most states differentiate between those who are nurses and others who have different hours than 40-hour work weeks and those who do have 40-hour work weeks.  In terms of her employment, is she under a contract or union? If so, she may be protected from such ultimatums.  Further, if she is not under a contract, she could be fired with or without cause. 

According to Section 448.01 of the Florida Labor Code,  ten  hours of labor shall be a legal day's work, and when any person employed to perform manual labor of any kind by the day, week, month or year renders 10 hours of labor, he or she shall be considered to have performed a legal day's work, unless a written contract has been signed by the person so employed and the employer, requiring a less or greater number of hours of labor to be performed daily.   Further,  unless such written contract has been made, the person employed shall be entitled to extra pay for all work performed by the requirement of his or her employer in excess of 10 hours labor daily.

Federal laws don't necessarily prohibit employers from enforcing mandatory overtime for those over 18 or 16 I believe.  Same holds true for state law.