He apparently didn’t reveal our past relationship.
College relationships are often messy. They represent growth periods for young people and do not always showcase behavior at its best. Unless he was dangerous or wildly disrespectful in ways that could be hurtful to your friend, keep those details to yourself. Share highlights, and if she wants to know more, tell her she should ask him.She is now married and raising the child as her husband’s. Her husband has no idea it’s not his child.
Contact your mutual acquaintance and talk to her, in person if at all possible. Tell her that you know the identity of her child and are concerned about what happens down the line. Describe the worst-case scenario in your mind — about your children, who are siblings, falling for each other without knowing the truth.