You obviously have no experience with how badly things can go with an uncooperative man involved. Your ideas are very naive.
First of all, “joint mature decisions” between adults who are in an unwanted pregnancy situation are rare, let alone when there are two teenagers involved.
Second of all, you get this kid involved and start “discussing” what to do, and she decides that she wants to give the baby up for adoption, he can stop her from doing that and petition the court for sole custody He can also choose a different adoptive family.
Third, he could potentially take custody from this fifteen year old girl if the circumstances were right.
Fourth, I don’t know what kind of person this guy is, but what if he’s very irresponsible. If you think the courts are going to keep the child away from him because he’s irresponsible, you have another thing coming. You give up a huge amount of control when you get into a joint custody situation. She cannot move, she cannot make decisions alone, and 90% of the time there is conflict between the families. Why get into that siutation if the guy doesn’t even know he’s a father yet?
Fifth, if her daughter decides to get married someday, she very well may not be able to let her husband adopt the child.
Sixth, he’s 18 or 19. He has no money. He can’t pay very much child support to begin with.
Your pie in the sky ideas about “the father being involved” are dangerous. Don’t take my word for it- talk to a good, experienced lawyer who specializes in family law.