Honestly, if you tell your friend something liek she's stupid or crazy, that will only drive her away from you and would probably only make her want to havea baby even more. True enough, it's definitely no easy task to raise a child at any age, let alone as a teenager.
You can keep trying to talk to her, but I can't say how much you'll get through to her. If she is indeed not pregnant, the best advice I can give you is to tell her to invest in an item called a realcare baby - they are rather pricey, but they are an amazing simulation baby doll. These things are even used in parenting classes because they're that good. If you stick your friend with one of these things for a while, she may chance her mind about having a baby.
Unless the parents of both your friend and her guy give consent, they can't be married until they are both at least 18...But they can be engaged. Who is to say this guy won't just get up and leave your friend after he experiences the reality of fatherhood? I hate to be so harsh, but I know there's not exactly a surplus of teenage guys who are willing to settle down with a family before they're even adults.
The best way to approach this is not to tell your friend how stupid she is (not saying you told her this, but just making a point). You must tell her how much hard work there is involved with child care, and how she'll be hurting herself. She'll need to put a hold on her education for a while, she won't be able to hang out with friends or have much of a social life anymore. All money won't be spent on things she wants, but things the baby needs - babies are very expensive. Something tells me that your friend believes that having a baby means nothing more than carting around a stroller...You are a mother yourself, so you already know all these things. The advice you could give her would be coming straight from someone who has had experience.
I wish you guys luck - I hope your friend makes the right choice.