ClawedGarnish has the right idea. If you randomly get an achievement then consider it a freebie. I have had it happen before as well, so it's not unheard of. Some games tend to occasionally unlock achievements even when the criteria has not been met, so
that's all well and good for you if you're able to get a free achievement. :P
it is a freebie, but just for your friend. it's a glitch. for some unknown reason the game thought he completed that task and unlocked the achievement. you get it in a few games. i know for a fact if you die in a certain way in assassins creed (1) then it
unlocks an achievement.
Edit: by unlocking an achievement i mean like collect 100 flags achievement not an achievement for jumping off a clif.