I have a set of Truvativ 5D cranks and I'm not impressed with the crank arm alloy. You can tell how poor it is from the pedal threads. I also have a set of XTR 960 cranks and the difference is significant.
Heh, I did something similar on my morning commute last Wednesday. Mine were 1994 Shimano STX cranks, so of course they probably weren't as strong to begin with, but I also weigh over 100 pounds less than your friend.