OK, I have a friend that had a child, but the father was not there to sign the birth certificate, the child does not have his last name, so they went to court for child support and a dna, and everything came back legit, but now the father name is on the birth certificate, how is that possible if they didn't have no type of aggrement on that issue?

An agreement doesn't need to be made between the two of them to put his name on the birth certificate.

Once he was proven to be the father though the DNA test his name automatically goes on it. It's isn't like when the child was born when you sign the acknowledgement of paternity voluntary, in this case he was proven to be the dad so he really didn't have to sign anything.