OK, I have to put my 2 cents in here. Just can't let a debate go by without saying something, now can I?
My mother was 16 when my older sister was born. So from that standpoint, I know that everyone makes mistakes. As long as this girl is trying to do what is best for her child, I see no problems. This doesn't make her a bad or troubled teen. this simply makes her a person who made a mistake early in life.
Second, my sister, brother and I ALL have DIFFERENT fathers. My mother was married to all but mine (Thank God!). I don't really think that has a whole lot to do with anything except maybe being a bad example. In my case, I didn't know until I was 9 that my brother's dad wasn't mine.
Lastly, I see no problem with them being friends. If something else developes, then will be the time to step in and talk to him about his and her responsibilities. Both to themselves and to the child. Becoming invoved with someone with a child is not something to take on lightly, no matter the age of the child or the parent(s).