If I never see the guy again, never play the game again (basically the problem postulated), I take any offer. But I absolutely don't let him know that ahead of time. Even if he gives me $0, I'll let him have all $5000. Unless that money came from a charity to feed orphans or something.
Good point. This game is designed so that the people discussing it will have a tendancy to lie about how they would really act: Potential players discussing the game are incentivized to provide the
appearance that they would deny small values of money, but are incentivized to accept all values of money when they are actually in the situation.
And you do whatever you can to convince him. Perhaps if he seems like he has a high sense of fairness. You could appeal saying "come on, we should split it half way, I'd do the same for you". If he seems weak or desperate, tell him you don't care that much, and burn a $100 bill in front of him to show him how much you don't care. It's all negotiating techniques and reading people at that point.
I think the most believable situation without pissing off the person making the deal is to start out at 50-50. If you try to demand more, he may just spite you. If he mentions A Beautiful Mind or Nash, I start making him think I'm irrational. How the second guy reacts to the offer is the least interesting part to me. Either you are rational or spiteful. What the first guy offers, and what the second guy tries to get is much more interesting.