I collected all 151 pokemon legally on the original Pokemon red

If you're American, then I'm calling bullshit, unless you were living in Japan at the time. Mew was not catchable in the original red and blue, and the only way to get it legitimately was to attend certain Japan-only Nintendo events, where a Nintendo official added it to your cartridge.

Here in the states, the only ways to get Mew at the time were GameSharks or trade, but unless you traded with someone who could get to Japan and was willing to give up their event exclusive Mew, it was probably just from somebody else he used a GameShark.