That would be an interesting case. Arguments against it being a contract would be lack of consideration (this assumes they weren't pooling their money to buy tickets) and lack of a writing (both for evidentiary reasons and as a potential violation of the statute of frauds). His friend's "are you jerking my bobber" statement suggests that the friend didn't view them as having an enforceable contract.
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.