Ok, the rules might be different in our countries and I'm not going to argue anymore. A quick Google search helped to word what I tried to say: http://definitions.uslegal.com/l/legal-father/
http://www.legalmatch.com/law-library/article/fathers-rights---effect-of-name-on-birth-certificate.html
http://resources.lawinfo.com/family-law/south-dakota/what-is-the-difference-between-a-legal-father.html
I would suggest them to consult a lawyer or something. Entering "unknown" or a different man as a father than who is on the birth certificate is going to create confusion for sure.
http://www.legalmatch.com/law-library/article/fathers-rights---effect-of-name-on-birth-certificate.html
http://resources.lawinfo.com/family-law/south-dakota/what-is-the-difference-between-a-legal-father.html
I would suggest them to consult a lawyer or something. Entering "unknown" or a different man as a father than who is on the birth certificate is going to create confusion for sure.