Talking to someone else's child about sexual health

I couldn't think of a better way to phrase this topic I apologize if it's misleading.. My niece recently had a friend over for a weekend sleepover. My niece who is 10, got her first period last year and happened to have it while this friend was over. This friend had seen my niece change her pad in the bathroom and said that she had been bleeding like that before and was afraid something was wrong with her- she also told my niece that when she told her mother about it, she was told that it was sinful to speak of those things and that it wasn't to be brought up again. Of course my niece was confused and brought the situation to my sister inlaw's attention quietly. My sister in law was in a moral dilemma over what to do- she felt sad for this little girl and wanted to explain things to her the same as she had done with her own daughter. However, she didn't know if it was her place to speak to someone else's child about these things. This little girl is my niece's best friend and my sister in law is afraid to even mention it to the mother fearing an argument that would hurt their friendship.