Couldn't he see if it will boot by selecting it as a startup disk in System Preferences > Startup Disk?
I can't remember which walkthrough I used when I replaced my hard drive, but it said to select your external as the startup disk and restart your system. If it boots into OS X, then it is ready to be placed inside the computer. Afterward, you select the internal drive as your startup disk once again and do the swap.
I can't remember which walkthrough I used when I replaced my hard drive, but it said to select your external as the startup disk and restart your system. If it boots into OS X, then it is ready to be placed inside the computer. Afterward, you select the internal drive as your startup disk once again and do the swap.