When is the right time to tell an adoptive child they were the product of a rape?

My best friend and her husband's adoptive daughter is 12 years old. She first suspected being adopted due to the obvious physical contrast so she was told the truth early on but not totally.

The missing part she wasn't told is her biological mother gave her up for adoption after giving birth. She was the product of a rape. She made an agree with the adoption agency to never ever be contacted. She doesn't want to be reached in any way at all. Meanwhile the dude served his prison time.

It's said the girl is the female version of the rapist in appearances. Now my best friend is concerned about the girl wonder about her background. She still wants to know who her bio mother and father were, if they are cool people, etc. When should she be told the truth about her birth? Or should she ever be told her bio mother never wants to see her because she was the product of rape? If you were the girl, would you want to know that?

When is the right time to tell an adoptive child they were the product of a rape?
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