Should I notify a father he has a child?

A girl I know had a child in her senior year of high school. The child was given up for adoption and now enjoys by all indications a stable and happy life. The father of this child doesn't know he has a child. The parents had split, and the pregnancy was kept a secret to everyone except the mother's immediate family. Now the child is six years old and the mother still has contact with the child, visiting it, and posting social networking statuses about it. I feel the father has a right to no about this, but I am hesitant to intervene because I don't want to cause unforeseen repercussions. I think if the mother didn't have contact with the child I would be more likely to be able to remove myself from the situation, but it angers me to see her hold all the cards. I consistently think about letting the father know, but I don't want to cause problems for him and his livelihood or the foster parents who seem like solid individuals and good parents. The mother, out of everyone, seems to be the least stable and responsible of all involved. It kills me to see public posts on facebook about the child from both the mother and her own mother. It just doesn't seem fair to the father. What should I do?