I probably wouldn't take that action.
First of all, you might as well just say that he's buying half of your action(ie. you are paying $50, he's paying $50, because if you lose....that's what you're paying, it doesn't matter if it's going to the casino/tourney or your buddy). Then on top of that you're giving him his initial money plus half of the winnings... The majority of the time for a beginning player(which I'm guessing you are since you're asking this question) you will not come out ahead here in the long run. If you're just wanting to play for shits and giggles, then this isn't terrible because you get off at half the buy-in, but don't go into it thinking you aren't paying anything, whether you win or lose, you're paying at least half the buy-in