What advise would you give.....

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This afternoon a friend of mine(girl) talk about her experience at her sister's(married) house. She went to her sister's house to spend some days with them, on the third day, at midnight, she woke up to see her sister's husband fondling are private part and boobs. All she could do was to raise an alarm, but unfortunately her sister didn't wake up, he pleaded and beg for forgiveness. That morning she pack her things and left angrily without saying a word to anybody. But now her sister is demanding to know why she had left without telling her. After explaining what had happen to me, she ask me that do she need to tell her sister or just keep it to herself. on saying that,i became speechless,should i tell her to explain what the naughty man had try to do to her to her sister or just advice her to keep it to herself since he has beg 4 forgiveness and will never do that again, if you were in that situation, would you tell her to let her sister know what had happen or just keep it to herself?