Swapping back and forth using p2p/DEX/CEX but this method involves fee.
If you choose this method, make sure to swap back to a newly generated address preferably using new wallet that differs from the old one by seed, passphrase, or derivation path. Also, be sure to mask your real IP when doing so.
Centralized exchange can be used but it does not guarantee privacy. It will still be seen as a poor method to do it. If he does not wants ti just coinjoin (decentralized mixer) he can use centralized mixer just as Merit.s posted already.
Or he can choose to first convert to monero and convert the monero back to bitcoin which is also good. He should use a decentralized exchange for it instead.