Brian's right.
Even a .500 isn't going to take down much of a tree in a single shot. If that's the bet, I'd save the purchase price of a big caliber gun I doubt you have a use for (or the experience for) and tell your friend he wins the bet. If the number of shots wasn't stipulated, get a good 38/357 revolver and learn to shoot while plugging away however many rounds your particular tree/s take.