The withdrawal shortcode for fiat coins is [wallets_fiat_withdraw].
If you have set up fixer.io access correctly, you should not be able to see the fiat coins in the standard [wallets_withdraw] shortcode.
You mentioned that the user does not have open orders on the exchange. Did you also check for pending transactions? You can go to Wallets -> User Balances to verify if the user has enough total and available balance for each coin.
with regards