Almost sounds like the ratcheting jack stands you guys were using were improperly manufactured. The set I have was one of those $20 auto parts store specials (included a floor jack too) and the ratchet mechanism has enough play to go to the middle of the shaft. They can get caught on the tip of a tooth and drop down a notch if they're wiggled, but from what you said (fell all the way down) it sounds like your friend had a faulty jack stand.