I may not stake in this situation.
1st, you don't know if he is a winning player or not.
the logic is like this
if he is a wining player and come to casino regularly, he should keep a very good bankroll management, so he is not lacking of funds.
and if he is a winning player, and not lacking of funds why he needs to take a part of win/profit to others who he did not know?
therefore the situation is he is not a winning player...
this stake is not a positive expectation...therefore I may not...
he may be in a up swing some times but you still win.
the 2nd thing, is if your funds are secured and able to keep a track on the win, and get your profit and investment back?
BTW: if this stake happens on buyin a big buyin tournament, I may reconsider his ability , style ,etc. to make the decision.