[Porsche-O-Phile]

Agreed, I believe there's not any legal obligation to return 1/2 (or any amount) to the donor especially if it's clearly a gift ("here go have fun"). If it's a loan probably standard contract law comes into play and the donor would be obligated to return the original amount. However the appropriate thing to do would be to hand a big chunk (1/2 is probably fair) over to the original benefactor of the money that was used to procure the winnings.

If I give you $2,000 to go invest in stocks and you happen to turn it into $20,000, it might be a different story since your knowledge and expertise had something to do with the success and is worth something - with gambling, not so much. Again, it would depend on the circumstances and particulars of the arrangement however.