If a woman was to ask for divorce from her husband without any reason then that's a very grave sin in the eyes of Islam. There has been reported a hadeeth in which Rasulullah sallallahu alaihi wa sallam states "Whosoever woman asks for divorce from her husband (without valid reason) then she will not smell the scent of Paradise "
A very serious matter indeed.
However of course a woman can ask for divorce from her husband provided that the reasons under which she is seeking divorce are valid under Islamic law, for e.g. , Her husband is Abusing her either Physically or Mentally, or He is neglecting her rights as a wife and not providing for her, Or Maybe he Abuses her.....in short, she has the right to ask for divorce from her husband if he violates any of the rights that Allah has given her, or neglects his duties towards her.