What are your ages?
Why did they break up?
What makes you think she won't get back with him?
I'm going to be frank with you. She may be saying this right now and she may even mean it right now, but with this baby coming she will think about it and its highly possible she will change her mind. Maybe she won't but chances are high that she might because they have a connection now where she will have to deal with this guy for the rest of her life whether she wants to or not. It's not uncommon for a guy to freak out when he finds his girl is pregnant, especially when they are so young, it's also not uncommon for the girl to take offense, break up with him, say she hates him and then forgive him.
I would suggest that if you really care for her, that you be a friend to her first before anything else. The more time you spend with her and the longer you know her and are friends with her, it will eventually become apparent whether she likes you 'like that' or if she just sees you as a friend, and also to see if she will get back with her ex.
Another issues is that she may feel very alone now, and she may cling to you for support, but you want to make sure that if you do get involved with her its for the right reasons and not the wrong, hence why I'm saying that you keep it as friends, until things sort out a little. If she plans on keeping the child, thats a huge responsibility on her, and you if you get involved.
Give it some time. Be yourself, don't always worry about what to do or say, just look at her as a friend and it will be a little easier to be yourself. You want to be yourself because if she does start to like you, you want her to like you for the real you. It's not an easy situation, but if you take things slow, hang back, get to know her and let her get to know you, then you will be able to see the situation a little clearer in a few months.
I welcome your thoughts, let me know if you want to talk more