Any chance you could ask if he has ever noticed a black mark on his macbook pro between the battery and the battery release buttons, right where his has popped!
Iv noticed a few times that i get this black sort of charcoal mark that wipes clean in that area and first thot it ws my carry case rubbing off against the hot battery... i have since stopped using the carry case and the mark is still appearing!!!
I cannot afford to have anything happen to my laptop or send it for repair/replacement as it has all of my degree work on it.
Also did the was the battery replaced or the whole macbook pro???
I had the same problem and I inspected the battery well and battery very closely. I didn't see a black mark like the one you describe (nor any kind of black mark or anything else out of whack.)
I only needed the battery replaced. The battery itself kind of "popped" but all the expansion was downward, away from the rest of the computer.
Now that the battery is replaced, my computer is behaving normally (even before the battery popped, it had been shutting off when the battery power when below ~10%).
FYI to all - check Apple's web site for battery recalls that may affect your MBP. My battery was actually under a recall, but I didn't know about it until after it popped. Link: http://www.apple.com/support/exchange_repair/. Certain models of iBooks, PowerBooks and and MBPs are under a battery recall.