If you knew someone's baby wasn't the supposed fathers, what would you do?

Clois
A little info, 
My neighbor who is constantly flirting with my boyfriend came to him saying how her 5 month old isn't actually her fiancées. It's his brothers. I guess she admits to consistently cheating on her fiancé. So, I feel like he has a right to know? She has no shame in it either. They also have a 4 year old together. But that baby is going to grow up not realizing her dad is actually her uncles, and her uncle is her dad 😳 what would you do?
(We live in apartment buildings and she constantly tries to hit on my boyfriend. It's to the point where he is uncomfortable to run into her if I'm not with him. He's completely disgusted with the behavior. He wants to tell her fiancé also but I'm worried about it not really being our business. But at the same time, I would want to know the truth if I was the fiancé.)

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