Why is complete consent listed as a requirement for an act to be a mortal sin? What does the complete consent, when listed as a requirement for a mortal sin, mean? 

Objection: It would seem that there are mortal sins without complete consent.


A woman goes to get an abortion because of fear from her husband.
Her consent is diminished because she does abortion through fear and therefore her act is not done with complete consent.
But still, her act is a mortal sin.


Therefore, it is would seem that complete consent is not a requirement for an act to be a mortal sin.

To be clear, the definition of the mortal sin (from CCC 1856):


  When the will sets itself upon something that is of its nature
  incompatible with the charity that orients man toward his ultimate
  end, then the sin is mortal by its very object . . . whether it
  contradicts the love of God, such as blasphemy or perjury, or the love
  of neighbor, such as homicide or adultery. . . . But when the sinner's
  will is set upon something that of its nature involves a disorder, but
  is not opposed to the love of God and neighbor, such as thoughtless
  chatter or immoderate laughter and the like, such sins are venial.
The objection is irrelevant.

A decision on a matter that serious does not happen in isolation.    


  CCC 1857 For a sin to be mortal, three conditions must together be met:
  "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."  


Put in the vernacular: so ya knew it was wrong and ya did it anyway.  

As to complete consent.   


  1859 Mortal sin requires full knowledge and complete consent.  It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice.       


This answer assumes that the person in question is a practicing Catholic in full communion with the Church at the time of this choice.   Any other case is irrelevant to the technical question you have on the matter of consent of the will.   

A brief summary from the Catechism.       


  1874 To choose deliberately - that is, both knowing it and willing it something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin.  This destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death.  


The choice for an abortion that you provide in your example was not the only choice available.  Any assertion of coercion or fear as being sufficient grounds to mitigate the act of the will is unfounded (at least in part if not in full) since the doctrine of free will presumes agency on the part of the sinner.  Beyond that, as a member of the faith community, there is available the counsel of the pastor and assistance from the rest of the faith community.  Choosing not to seek pastoral counselling is hardly a mortal sin, but it's certainly an error in judgment.  (Errors in judgment provide fertile ground for sin.  Don't I know it.)  

As to imputability

(Ref: Catholic Encyclopedia) 


  When the cause produces a twofold effect, one of which is directly willed, the other indirectly, the effect which follows indirectly is morally imputable to the sinner when these three conditions are verified:
  first, the sinner must foresee at least confusedly the evil effects which follow on the cause he places;
  second, he must be able to refrain from placing the cause;
  third, he must be under the obligation of preventing the evil effect.   


He's on the hook, as is she.    

Beyond that, Jesus taught us: Be not afraid.  

About the role of the will in Mortal Sin

From St Augustine:  (Reply to Faustus XXII.27) 


  ... something said, done or desired contrary to the eternal law, or a thought, word, or deed contrary to the eternal law.  This is a definition of sin as it is a voluntary act.  (Catholic Encyclopedia)    


Lastly: the condition of being in mortal sin is not permanent.

The sacrament of confession and reconciliation is in place to return the pennitant into communion with the Church.  The help's there, take advantage of it.   

The edge case ~ suicide

In recent years, the Church seems to have adjusted its hard line on the suicide as a mortal sin.   


  2282 If suicide is committed with the intention of setting an example,
  especially to the young, it also takes on the gravity of scandal. 
  Voluntary co-operation in suicide is contrary to the moral law. Grave
  psychological disturbances, anguish, or grave fear of hardship,
  suffering, or torture can diminish the responsibility of the one
  committing suicide. 


It's not a get out of jail free card, but it opens the possibility of mitigation, or a conclusion that the consent of the will was significantly impaired.  With that said, the Church then leaves the fate of the suicide in God's hands:  


  2283 We should not despair of the eternal salvation of persons who
  have taken their own lives. By ways known to him alone, God can
  provide the opportunity for salutary repentance. the Church prays for
  persons who have taken their own lives.  


The Church will not condone suicide, but it need not condemn the suicide as a reflex.  This leaves open the provision that, in the case of suicide, a defect of the will may overwrite "complete consent" or even sufficient consent.  (per CC 1859)