Before you start, look at the back of the new hard drive, and set the jumpers on it so that it is set to be a slave, not master.
Look inside your PC, find your existing hard drive.
It will have a ribbon cable going to a connector on the motherboard. There should be a spare connector 1/3rd of the way along that ribbon cable.
You screw the new hard drive into a spare bay, and connect the spare connector on the ribbon cable to the new drive. You then find a spare power connector, and connect that to the new drive.
Finally, when you first power on with your new drive in place, enter the BIOS, and set the BIOS to auto-detect all drives.
Your mate is mistaken if he thinks he has his drive working without a power connector.