The woman has committed a very grave evil, in that she is complicit in the death of an innocent human being. How much of that evil can be "imputed" to her, that is, treated as "her fault", may vary according to circumstances.
Catholics recognize two broad classes of sin: mortal sin (also known as serious sin), and venial sin. The distinction between the two is that although both are "disobedience, a revolt against God" (Catechism of the Catholic Church, paragraph 1850), mortal sin is of such seriousness that it destroys charity in the heart and requires a new outpouring of God's grace to the sinner (paragraph 1855 of the Catechism), whereas venial sin "merely" wounds charity, but does not turn the sinner away from God entirely.
As elsewhere noted, the Church considers sins mortal if:
- They involve "grave matter"—a very serious offense against God. Complicity in an abortion certainly constitutes grave matter, since "from the first moment of his existence, a human being must be recognized as having the rights of a person—among which is the inviolable right of every innocent being to life" (Catechism paragraph 2270)
They are committed with "a consent sufficiently deliberate to be a personal choice" (Catechism paragraph 1859). This may or may not be the case for all abortions; often, it seems, women feel themselves to be, if not coerced, then at least with choices restricted. From what I've heard, very few women have an abortion because they really want to.
They are committed with "full knowledge", that is, with "knowledge of the sinful character of the act, of its opposition to God’s law" (Catechism paragraph 1859). This is another requirement which may or may not be fulfilled in particular instances. On the one hand, if a woman isn't aware of the Catholic Church's stand on the issue, one could argue that she does not have this sort of "full knowledge". On the other is the statement in the Catechism that "no one is deemed to be ignorant of the principles of the moral law" (paragraph 1860)—including the proscription against taking the life of an innocent person. The question would then need to be answered, to what extent the woman believed that she was taking the life of a person.
Even if the woman might be deemed to have committed a mortal sin (a matter between her and God), she is in extreme spiritual danger, but she's not automatically going to Hell.
What she is, automatically (at least, if and only if she's Catholic), is excommunicated.
So what is an excommunication? The Catholic Church considers itself to be a community of believers, and by committing certain gravely wrong acts, people can demonstrate themselves to be (or on occasion can be declared to be) so different from the community of believers that they're no longer part of the community, nor able to function as one of the community—hence, excommunication.
Canon 1398 of the Code of Canon Law states:
A person who procures a completed abortion incurs a latae sententiae excommunication.
This means, an automatic excommunication without requiring a declaration from an Ordinary (that usually means the local bishop).
Canon 1331, Section 1, states:
An excommunicated person is forbidden:
1 to have any ministerial participation in celebrating the sacrifice of the Eucharist or any other ceremonies of worship whatsoever;
2 to celebrate the sacraments or sacramentals and to receive the sacraments;
3 to exercise any ecclesiastical offices, ministries, or functions whatsoever or to place acts of governance.
In practice, what that means for the woman is first of all that she is forbidden to receive Holy Communion,or get married, or be confirmed, but not forbidden to be present at Mass. If she has any of the lay ministries of the Church (for example if she is a lector or an Extraordinary Minister) she is forbidden from exercising those ministries). She is also, as it appears, forbidden from doing such things as saying the Rosary (things known as ["sacramentals"]).
Canon 1323 does allow for exceptions to this rule as well. The woman will not be excommunicated if she is younger than sixteen years, or if she was coerced physically or psychologically, or if she did not know that in having an abortion, she was violating a rule of the Church (this last appears unlikely to me).
To regain communion with the Church, the standard (normative) requirement is to go to Confession (also known as "Reconciliation" or "Penance"), and confess her sins, and receive absolution.
Generally speaking, the Sacrament of Penance/Reconciliation/Confession will absolve you from all sins, mortal or not (assuming you're honest with the confessor, and try to confess all the serious sins you remember). But abortion may be different. As I read the Code, it's possible (but not clear to me) that the absolution needs to come from the local bishop. I'm not sure how that would be handled in an ordinary case of Confession.
There are other (extraordinary) means of being forgiven mortal sins. They require, at a minimum, perfect contrition—that is, being sorry for one's sins not because of fear of hell, or some similar reason, but specifically because one has offended against the goodness of God (Catechism, paragraph 1452). These extraordinary means of forgiveness can operate up until the very moment of death.
So: Could it happen that a hypothetical woman who has had an abortion goes to Hell? Certainly; if the woman is entirely aware of the gravity of what she has done, and realizes that it is wrong in God's eyes, and does it anyway because she wants to, and fails to ever repent of it even up to the moment of death, such a thing is possible. But the mere fact that she has had an abortion doesn't mean she is hell-bound. The Church does its best to reach out to women who have had to go through this and support them.