He's Always Known
Finn might not have the best grades in the whole school, he might've been hit in the head one too many times on the football field, but he's not stupid. He knows… He knows everything.
He knows the baby isn't his.
He knows who the real father is too.
But does he care?
No.
He couldn't lie it bothered the hell out of him when he first figured it out, but when he saw that baby girl on the sonogram he no longer cared that his girlfriend and best friend had sex and conceived a child, he loved this child enough not to care anymore. At first he was going to tell her that he knew but now that she had been pitilessly kicked out of the house by her bible beating father and drunk pliant mother he couldn't do that to her. It would crush her.
Maybe someday when their little girl had grown up and had siblings she would confess, but it wouldn't matter anyways.
He's always known.