I’m pretty sure there have been a number of cases where the less fortunate friend has tried to enforce such a mutual promise with the mutuality in understanding that they both will be purchasing tickets half for the other in the future to be the recipient being the consideration. I imagine those cases largely have turned on differing recollections as to whether such a pact has been made. Not sure the written part would be so important if they agree on and attest to the same details? And as to the friend’s expression of shock, that could be simply of the winnings, were he not aware of them—or that the friend was volunarily coming through on auch a contract. As a longtime friend the beneficiary may have already thought that such attempted enforcement on his part wasn’t worth losing the friend.
Yes, it would be a highly fact-based case. I don’t think the IRS would care too much about this case, though, because the unified credit is so much higher than the amount given to the friend. Of course, if the winner was in a significantly higher tax bracket than the friend or had a very high net worth aside from the winnings, the IRS could take the position that this was an attempt to shift income tax or estate and gift tax obligations to another in order to reduce the overall taxes associated with the winnings.