Can I request a PC version of a redeemed game after putting in the code?

He can try to refund the code, but there is no way to change platforms or stores

So codes are refundable??

If the code was already redeemed then no it wouldn't be refundable.

If they gifted you the Xbox version of the game, you won't be able to install it and your friend would need to request a refund.

If they gifted you the PC version, you can redeem the code at http://redeem.microsoft.com and install it through the Microsoft Store app.

The PC version of Fortnite has it's own launcher and isn't connected to the Xbox store nor the Xbox network.

Thus the code can only be used for the Xbox version of the game.

After redeeming the starterpack and binding your Xbox and Epic account, you can use the starter pack on your PC as well.

Codes are redeemed the same way regardless of if they are for console or PC. If what you redeemed was for the console version of the game, there's nothing that can be done.