You are NOT the father

So, a friend of mine has a 15 year old daughter who believed her entire life that my friends husband is her biological father. As it turns out, he entered her life when she was 3 months old. Now, she's doing normal 15 year old things, like not telling her father about her boyfriend and they are in fear that she has figured out the truth. They have not had a conversation with her about this, but they are very upset about what could happen if she finds out. The question I have for you, is would you tell her, or would you keep quiet and hope she never finds out? Apparently both families know the truth and a few friends. it's unclear what's written on the birth certificate as this revelation came out during a conversation over several cocktails and that question was never asked. Other details, they also have a 12 year old daughter who is his biological child, and the 15 has her mothers maiden name as her last name. *Please enjoy this insensitive gif and know it's meant to lighten the topic not judge or make fun of it.*