It is BOTH of their responsibilities if the baby is born.
Only if he originally said or implied he had any intention of sticking around. Then she has a case to argue that he made a commitment and needs to own up.
If she chooses to have a child outside of a committed relationship, then she needs to own up to that decision. She has every opportunity to avoid becoming or remaining a single mother. Innumerable birth control methods, adoption agencies, Safe Place, abortion services. If her choice is to become a single mother, then she needs to grow up. Old enough to have a baby, old enough to be a mom. Lose the helpless little woman act.