Should I tell my friends sister that he said he dated an 11 year old girl?

This was in 2013 when we were both 19. I forgot why he brought it up, but he said that the youngest girl he dated was 11. I have no idea how old he was when he dated her. I kind of freaked out on him when he told me, bur he kept denying it was wrong. I never really thought about it until a couple of years ago, and asked if he remembers telling me that. He said yes. When I asked him if he thinks it's wrong, he said yes. Well, i told his mother anyway who is his legal guardian. She was slightly upset that I waited so long to tell her, but she thanked me for telling her
The reason I even said anything is because his sister has a young daughter that he's around often. Close to a year ago, I decided to tell his sister. I know her a little bit. She works at Walmart and I say hi when I see her. I'm not even sure if she knows who I am. Anyways, I messaged her about it on Facebook. She never responded. I'm not even sure if she read it. I feel she really needs to know, so I was thinking of finding her on a day she's working, and going in and telling her. I don't know if that would be a bad idea, or not. What do you think?

Should I tell my friends sister that he said he dated an 11 year old girl?
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