It should ideally be a Bitcoin Core wallet ran over Tor, but if not, then it should be a brand new light wallet, preferably a client he has never used before, also ran over Tor.
While you're right regarding privacy (the question of this topic), let's not forget security. I'd always prefer some sort of cold storage, be it an airgapped offline laptop, a SeedSigner seed or a good open-source hardware wallet.
Since he already has a hardware wallet, the
ideal way for him might be wiping it and transferring the funds back to it afterwards. Before doing so, he should set up the wallet software to run over Tor and / or connect to his own Bitcoin Node through the software. That would be my recommendation.
If the hardware wallet doesn't support wiping and creating a new seed, I'd probably buy a new one
(but I realize I might be in the minority, almost collecting these little gadgets..) or creating a fresh, independent wallet using a passphrase. It could even be as simple as simply 'btc' for the purpose in question here. Even write it down onto the device if needed!
It still depends on the software implementation, but my best guess would be that if you start the software and enter the passphrase on-device, it won't even (be able to) query the 'default wallet' balances.
It's possible that the software does cache the different xpubs though, so to be safe I'd check the wallet's codebase and also wipe the installation off the hard drive & reinstall it. Or maybe even switch to another software.