The fact that it is her legal right shouldn't weight too much in your decision.
In Nazi Germany, what Hitler and the SS did was their legal right. Civil law is, unfortunately, not synonymous with moral law.
Bonnie is right. If the "fetus" is a human person then abortion is murder, even if it is legalized murder (just like the Nazis legally murdered the Jews), and you can simply use a counter-example involving some other human person:
Imagine that Mr. Black is going to smother his grandmother to death on Wednesday and he informs his best friend, Smith, about his murderous intentions that Monday.
It seems fairly obvious that Smith has the obligation to try to prevent Mr. Black by persuading him not to and, if that fails, by going to the police and informing them of Mr. Black's murderous intent.
Saying "But what if the police arrest him and send him to jail?" would obviously be inconsequential. Likewise, saying "But what if Mr. Black's father (the grandmother's son) upon finding out about his plan to murder his grandmother tries to get revenge on his son by murdering him!!?" is obviously inconsequential. It's useless speculation, which, if consistently applied, would thwart us from doing almost anything we know to be morally praisworthy because there may always be some unintended negative consequences (if I help an old lady cross the street, she may get mugged!).
I think you have the obligation to try to persuade her to not get the abortion. If that fails, you should inform her parents or anyone else that you think may be able to prevent the murder from occuring. The fact that it is someone's choice to abort (murder) a child has no more significance than saying it was Hitler's choice to murder the Jews.