I hate to disagree with your mate but I believe he's in the wrong. The Mac doesn't care what is in that physical space inside the machine, it pulls the information as to what the device is based on, well, what the device is. When it attempts to communicate with whatever is connected to that particular SATA port it determines what the device is based on the response it gets.
Basically, if it powers up that SATA port and an optical drive / disc drive starts communicating, then it identifies it as an optical drive. If it powers up that SATA port and a hard drive / SSD starts communicating, then it identifies it as a hard drive / SSD.
I outlined setting this new drive as a boot location in Can it boot from SSD in optical bay caddy? about a week ago. Basically once the hard drive is installed, and assuming you've already moved your OS or installed an OS onto it, you will instruct the Mac to boot from this new hard drive vs the original hard drive. You can do this by following these instructions:
- Power the Mac on while holding the "option" key on your keyboard
- After a few seconds a gray screen should appear with one or more hard drives visible. Any attached hard drive that has a bootable operating system will show here.
- Select the desired hard drive and click "return"
- Once the Mac boots to the operating system, open System Preferences
- Open Startup Disk
- Select your desired startup disk
- Click restart to finish the process
Apple KB article detailing how to select startup disk
Hope this helps you out! Perhaps you could wager a bet with your friend and get a free drink out of this when it works.