NAME CHANGE AFTER DIVORCE

Does anyone know the law or procedure about changing your name after a divorce. To make a long story short, I just found out that my fiance`s ex wife still goes by his last name. They have been divorced for over 3 years. This is not something that is automatically done when you get a divorce? I know I shouldn`t let it bother me, but still aggravating. Can she just keep his name forever? To make the situation even stranger she has a serous boyfriend with whom she has a kid. Does anyone know anything about this, or have any advice?
Thanks in advance.