I am amazed at how laid back about the whole thing so many of you are. I have a 15 year old DD and I would absolutely support her every step of the way through a termination. It would be the only possible option.
No, I wouldn't leave her to decide for herself, by herself - that would be an absolute abdication of my parental responsibility. My 15 year old might think she wants to have a baby (she doesn't think this, that I know of!) - but at this point she also thinks all kinds of other crazy shit that I wouldn't entertain.
As the adult here, I know that having a child at 15 would be undesirable for her for a million different reasons. And I'd end up looking after both of them, wouldn't I?
Just no. Getting pregnant at 15 is a mistake, we all make mistakes - but no need to turn it into a lifelong mistake. My role here would be to help her think clearly and see it in undramatic terms. To access a termination, have it, and put it behind her.
Yes, I realise the OP said that termination isn't possible. In which case I'd swear mightily. But in general terms, this is my view.