Anything is possible....its 1 unit out of a few million. If you look around your house right now I bet every single piece of electronics you own.....somewhere, someone else has had major issues with that same exact product.
I call BS on this one. The computers USB ports doesn't put out enough amps to cause this type of damage only enough to charge at low current. He probably hooked up the i4 to wrong Old Apple Wall Charger.
This is just my opinion....
My guess is that a foreign (conductive) object got stuck in the port on the phone, then the guy (or girl) unknowingly plugged it in like most of us do nightly. What resulted was a short circuit. Even though the USB cable does not carry high amperage or high voltage, a short circuit is strong enough to melt plastic. Since there is no overcurrent protection on the cable, I'm fairly certain the short would be sustained long enough to also fry the phone.
To prove this theory BS , I went ahead and got an iPad charger (which is stronger then the iPhones) ripped a iPhone dock cable and shorted all wires together into a nice knot...the cable heated up alittle but not enough to melt it. The adapter got hot till it damaged...after that I immediatly plugged it to the iPhone 4 charger and waited some more...no melt...
My guess is that it has to do with RF leaking onto that bottom band and creating a short of some sort or just heating up the metal like a microwave. That's the only logical thing I could think of to make a ground-to-ground fire.