I would definitely use the past perfect there.
1) "He asked her why she didn't tell him all about it before?"
OR
2)"He asked her why she hadn't told him all about it before?"
Both are correct? Which one is more usual to your ear?
I would definitely use the past perfect there.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
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