Okayy I have a friend and she is scared and is wonering what happens if a 17 year old boy has a baby with an 18 year old girl? And whose last name would it have?

Probably nothing as far as the age goes. The baby has the last name that she gives him/her.

Physically, an 18 year old girl should have no problems with the pregnancy. Get proper care and take the folic acid etc, but her age should not be an issue.

Normally, the baby has the same last name as the mother, unless there is a reason to give it a different. I didn't change my name when I got married, but chose that the children would have the same last name as my husband. We decided this because we thought it would be easier socially, but it turned out to make no difference at all.