OK, annap, I guess you want a serious answer, huh? I think I'd wait and not make a decision until the situation happened and I weighed all the factors - I'd use fairness and giving the other person the benefit of the doubt as my principles.
However, I think it's far better to do things to avoid conflicts like this. If you're in a conflict, it doesn't really matter who's morally right - somebody's going to get hurt, and a relationship is going to be damaged. Better to use the judgment needed to avoid the situation in the first place - I think that's the morally correct thing to do. Let him get his own dang tickets, or have an agreement in place ahead of time and stick to that agreement.
However, if I agreed to buy somebody a lottery ticket with specific numbers and I didn't buy one with the same numbers myself, I guess I'd feel bound to give the other person all the money, and let that person decide whether to give me a cut. If you're in a community-property relationship where you're sharing assets with this person, then the split would be 50-50. Like I say, there are lots of variables involved, and it's probably best to avoid conflicts.
-Joe Offer, who'd still like to have that party-