he needs to sell his btc.
at op if you are usa based coinbase cash - usd is fdic insured.
I do not know how much you are cashing , but lets pretend it is 15,000 usd worth of coin.
send 1,000 once it hits sell it keep it in usd - cash not any coin of any kind.
that is now FDIC insured (assuming you are USA based).
so do this 15 times 1000 each time. you never risk a large percentage of the coin.
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OP needs to keep in mind that if he does do it this way to make sure that his Coinbase account is fully verified with the appropriate documents (ID, proof of address, source of funds - not necessarily of his Bitcoin stash but like a salary invoice or something) so that they don't give him any trouble when he's transferring the money out to his bank.
As well as making sure you pay all the appropriate taxes so you don't run into other kinds of trouble.