Well honey as much as I agree with you on the abortion part, there's not much you can do to stop her. She's the one who's pregnant and it's up to her and her parent's weather that happens or not. I'd advise you not to get involved even though you are her friend and care about what's happening. If you get involved you might complicate things way more.. Now, I know that even though I told you not to do it you might do it and if you are going to get involved think about the consequences and how to do it on a mature way.
If you get involved:
She's probably trying to find the easiest way out of this mess; At 13 you are still a child trying to grow up and perhaps she started to grow up to fast and now things are just over her head. Put yourself on her position, things are unclear and her whole future is marked by a wrong decision made 3 weeks ago, something that didn't even lasted as much as the pain is going to create. Her head is probably going as fast as a car ca go on the highway, way over the limits. There's no point to start fighting about what happened now because that's not going to make it go away, but you could try to talk to her in a friendly none judgemental way and expose to her that there's other options - now what she does with those options is not yours to decide - By giving her more perspective at least you'll know you did try. Don't make assumptions about what's going or trying to make her do things because she's already having heart time. Mainly just be her friend and support her (I know that this might be hard because what she wants to do goes against your principles. You are her friend and she needs you right now, be there for her. In the future if you think that you might not want to be as close to her as you are now well you could do it but never close a door or judge somebody for something they did because deep down they are probably judging themselves even more. Don't make it harder for her.)
Good Luck. PM if you need to talk or anything.