I'll preface all my remarks by noting that I, too, am a guy; I'm simply commenting on my experience of these situations, and will not pretend to know authoritatively what's actually going on inside any woman's head, or what is appropriate to say in all situations. Neither am I a trained counselor or psychologist, although I was an RA for a year, which required some very basic training in crisis counseling. You have been disclaimed.
Having been in a similar position more times than I care to count, all I can say with universality is that there is no correct answer.
Part of it depends on what her feelings are about abortion. Some talking heads would like us to believe it's strictly a medical decision for the woman; if she really believes that, then there really isn't anything that's worth saying; "I'm sorry" carries the kind of moral implication that she might find offensive, making it ill-advised unless you're both chomping at the bit to have a debate. However, it has been my experience that most women understand instinctively that there's more to it than that, and as such depression is a common reaction. She may be looking for support and comfort, someone to tell her that she is not the most heinous anti-mother in the history of the species. "The worst day of my life" has been a common phrase in my experience. In those cases, and I believe (assertion without proof) this to be the significant majority of cases in which the woman would actually tell someone with whom they're not on intimate terms, "I'm sorry" can be a good starting point - the ambiguity of the phrase in american parlance is helpful, in that it expresses (in a non-specific way) a sympathy for the difficulty (and even traumatic nature) of the experience, regret that a situation even arose for which this was a potential recourse, and so on.
Of course, I say "starting point" assuming this conversation should go somewhere - which may not be the case. How much is appropriate to be said will depend upon the depth of your friendship. The wisdom of getting into what could easily become a kind of grief counseling session with an acquaintance from calc I is questionable at best. The whole college dorm culture can be rather dangerous in this way, in that there is often something of a shortage of friends with long track records with whom it would be relatively safe to share such things, and lacking such an appropriate venue, people will sometimes reach out to anyone who will listen. This is both dangerous for them and you, so tread cautiously, and take advantage of the university's counseling center early and often.
BTW, here's a tip for all of you argumentative noders out there: odds are, if she knows you well enough to be sharing this with you, she also knows (or at least has an idea) of where you stand on the legality, morality, and ethics of abortion, so turning to a discussion of that will not generally be very productive. Few things will harden someone against you quite like an assault with logic when they're emotionally vulnerable. On the flip side of that coin, when engaging in public debate about aforementioned legality, morality, and ethics, nothing brings reasonable discourse to a screeching halt quite like standing up and shouting "How dare you judge me?" - think of it as a kind of Godwin's Law for public debate on the matter.