Format your SD card while it's connected to PC with card reader.
Put the file on SD card - it should be the only existing file. Name it correctly. If you don't have extensions showing in your OS - name it PD15IMG and not PD15IMG.ZIP. If you don't know - try once with and once without the .ZIP in the name.
Put the card in your phone.
Boot with VolDown+Power.
It should ask you for update.
If it doesn't - it means you can't follow the guides because you keep misunderstanding something, so give the phone back to your friend and advise him to take it to someone who can.