You can also use iancoleman's BIP39 tool to extract the private key based from the "master private key" (xprv) and derivation path included in the descriptor.
It seems iancoleman's BIP39 tool was last updated on 2021[1] and there's no support for BIP 86 which about deriving key P2TR. Have you tried extracting private key from P2TR address with this tool?
Just need the private key from the correct script type.
You don't necessarily have to derive the correct address, he just have to type the correct path
m/86'/1'/0'/0 in BIP32 tab and the private key list below should correspond to his TapRoot addresses.