To clarify -
1) The plan is to sell the ball and keep 51% of the net, 49% goes to his friend with whom he was at the game
2) The reason that he might be required to pay taxes is that he has taken investment ownership of the ball and that money can be viewed as income of the fair market value of the ball. He could sell the ball at that FMV price and wouldn't owe taxes on it. But, if the ball sold for more or less, he would owe taxes or could take a write off against the difference. No different than getting stocks or whatnot. Think of it this way - when you buy stocks, you already paid taxes on your income. When you sell the stock, you either take a capital gain or capital loss (this last week it was more loss than gain😉). Same thing here. What really would irk people is that throwing it back wouldn't be a qualifying tax deduction - he could still owe taxes.
The problem is that memorabilia isn't always sold, so it is possible that it would hold no investment value to the owner - if it was held as a reminder of the game (think of autographs that kids get at ballgames), there is essentially no value difference between that ball and any other that could fly out of the park. But, as he plans to sell this ball, this is irrelevant.
While the this is fun and exciting, remember that logic is occasionally used by the IRS (see Mark McGwire, #62).