My step-brother had a baby with his former gf when they were teens. She wanted to give the baby up right away, he wanted to get married. They didn't get married, but kept the baby. Before my nephew was a year old, they split up. Both of them are drug addict losers who don't have any intention of coming back for him. My mom has been raising him since he was a baby, now he's almost 9. She got legal custody after about a year. This poor kid has severe emotional problems. He's been diagnosed with Attachment Disorder, after the school district shrink tried to label him ADHD. We don't know where his mother is. We know where his father is, but he NEVER calls or visits him. He shows up once or twice a year, but not to see his child. Probably the worst part for this poor kid is knowing that his dad has other kids that he is raising, but won't come back for him.
I certainly wouldn't presume that this might happen in your situation, but I do know that if my nephew would have been given up for adoption when he was born he would be growing up with a mother and father that love him right now, and would be much better off emotionally. My parents would like to adopt him, and have been told by his counselor that it would be really good for him, but since they can't find his mother, it will probably never happen.
I hope that since this girl is so determined to keep her baby, she gets some stability in her own life so that when she finds out how hard it is, she doesn't just abandon him/her like these people did.