I assume that, in this case, the defendant was accused of murdering a third party and that she claims she did so because her boyfriend threatened to harm her unless she carried out the killing. If this is what happened, the defendant would not be able to successfully claim duress as a defense for her actions.
It is true that a defendant is, in general, not responsible for his or her actions if he or she is forced to commit a crime by some other person who is using threats to coerce the defendant to commit the crime. However, this rule does not apply in the case of murder. It only applies to lesser crimes.
As the link below states (bold face added):
Except with respect to HOMICIDE, a person who is compelled to commit a crime by an unlawful threat from another person to injure him, her, or a third person, will generally not be held responsible for its commission.
Because of this exception, the defendant cannot assert the duress defense in this case.