Well, they can't give you the private key (at least, not without knowing the seed phrase, which you probably shouldn't give them). What they can do is tell you the derivation path... Or even better: give you a step-by-step guide on how to recover their BSV wallet on electrum for BSV.

If they give you a step-by-step guide on how to do this, the rest should be a simple as opening said electrum for BSV wallet, going to the menu "wallet" => "private keys" => "export". Then looking through this export to find the private key whose public key hash is 1Na9ZvogrWGWSYAzpWPFS5axKV4z7kQJTt, copying this private key and using it inside an electrum for BTC wallet.

But yeah, if they don't want to give this info to you, things get rather difficult. They could have chosen any derivation path... But don't get ahead of things, first try if they aren't willing to give you the info you require in order to recover almost $3500 in BTC.