Do you advise he sells and bail his kid sister? Everybody is confuse.
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Kidnapper can not demand for crypto since they didn't mention they're after your coins, also crypto transactions is traceable to your identity, you don't expect people who hiding their identity to ask for
bitcoin that contains details information of the receiver.
By details information, I guess you mean public address because I really can't think of any other information you're referring to.
Do you even understand how bitcoin work? Do you know anything about decentralization? Is seems to me that you lack the knowledge of decentralization...
If the kidnappers have some knowledge about Bitcoin then they should be able to complete their operation without doxing themselves.