Honestly, I would do everything I could to encourage the girl to give up the baby (as right now I have a 10 yr old son) I've thought this through when my sister had a pregnancy scare at 15 (a year ago)
I have been a single parent now for 10 years, and even though I was 22 when he was born, I know how difficult raising a child is. How much I've had to give up for him. I know that even if the father says they will be there to help, that doesn't always make it so. And at 12, there is no way that I could expect the couple to stay together to raise the child. I would possibly even offer to adopt the child, so that the parents could continue to grow as teenagers.