The most important thing to note here, is that if you invest 100% you will not get 100% of winning.
That said, you should work out a fair deal.
Investing 50% to receive stake back +40% of profit seems very fair. (using his 25% stake for 20% return logic).
Remember, this should always have stake back FIRST, and then the percentage.
Meaning if you invest 1k (half buyin) and he walks off with 10k, then you first get back your 1k, and then 40% of the 8k= 3.2k , so in total you now have 4.2k (in this case it works out to a total of 42% equivalent).
I agree with the poster who said that if you're going to be staking him often, that you should agree on makeup- if he loses buyin, it comes out of profit first next time, and then the stake return % is calculated on it..
Finally, just as a note to you: I would be very wary of this deal.
You say he's a great player and has proven himself, but doesn't have the money to play at these stakes.
Has he proven himself on these stakes? How often has he played on these stakes? Has he ever played with someone else's money (not borrowed, but staked)? Most of these factors may cause him to play 'scared money' and therefore not at his best game- plus, there's likely to be better players as the limits go up.
You say the 'local' games are really aggressive- how would he fare in this situation? How would he fare in a very aggressive game? Can he adapt?
'Local' implies that there are regs- and when there are regs, there are sharks- as well as fish. The local game by me is typically 8 handed, with 5 sharks, 1 fish, myself and an 'average' player, who's just a rock.
What kind of players are at this game?
Last, but not least:
You're staking him for a game he doesn't normally play, for a type of game he probably doesn't normally play, versus regs on a higher limit- so there's a good chance there will be good players there as well.. and all this...
With ONE buyin!?!
I never go to a live game with less than 3 buyins. Everyone can make mistakes, and everyone can get coolers. One 'small' mistake or cooler of QQ vs 2 pair or KK/AA on a low board, at the very least, will cost him 30-40% of his stack. Where does he go from here?
If you're only staking part of his buyin, then he's probably only going in with one buyin..
All of that aside, your money is at serious risk.
And if you plan on staking him for 2 BI, then be prepared to expect more of a % back... i.e. if you and your friends stake him for a total of 2BI, then you should be receiving 85-90% back overall (If you stake 1 BI out of 2, then you should be getting 42-45% back.. also since you're essentially staking 50% of 2 separate BI).
Anyway, staking players isn't as simple as it seems.
Do your research, get some friends to stake 2-3BI and offer him 5%-10% of his profit until he proves himself- if you think he has a good chance at winning.
If he's a good player, then 5%-10% can be a lot of money. Again, previous example: You and your friends stake 3BI at 2k each= 6k, he walks out with 10k plus the other 2BI that he didn't touch...
You get the 6k back, and 90% of the 8k profit = $7,200.
He walks out with $800, which is what he would be making on $200NL if he walked out with 5BI ($1,000) - except here, his money isn't at risk.
Then as he proves himself, you can allow him to keep up to 20 or even 25% ...