Can you post a pic of it?
If it is somewhat exposed try a pair of small vise grips. My favorite tool is most often a pair of needle nose vise grips.
If you cannot break it free due to clearance issues, seriously think about having a shop look at it or have your buddy live with it.
One more thing, is you need to at least try using a hand impact driver on it, even if it is hollowed out. Find a bit that can fit and then tap it in, then put it in the driver and give it a rap. When you are already hosed it usually is an exercise in futility as it is usually not that successful but it does work enough to try anyway when you have nothing else to lose.