[M.B.E.]

Suppose a player has 106% ROI in a $1,500 WSOP event. A backer pays the whole entry fee, with any prize being split 50/50. As the tournament starts, the total equity of the two of them is $3,090, of which $1,545 is the backer's share. So the backer has EV on the deal of +$45. The player has EV of +$1,545 on this transaction, but her EV would have been +$1,590 if she had bought in to the tourney herself without the backing arrangement.

If the player is broke then this is a terrific deal for the player; from the backer's perspective it is +EV but he should be able to negotiate much better terms.

If the player has a huge bankroll relative to the buyin, then it isn't a good deal for the player, because why give up even $45 of EV? For the backer it would be a fair deal.

In any event ZeeJustin is correct that if the player has ROI greater than 100%, a deal in those terms is +EV for the backer. But whether that makes the deal "fair" depends on other factors.