Assuming no previous agreement, I think I'd give him $1K.
My reasoning is my not having $50 in my pocket going into a casino would be a simple oversight, forgetfulness, and I think a friend spotting me $50 would not be a big deal to him or her either.
It would be like lending me a lighter (if I still smoked), or spotting me lunch money at a place that only took cash. (Though in the case of lending small amounts of cash, always with the expectation that it will soon be returned.)
On the other hand, $1K would be a nice windfall for me or my friends. It would be appreciated if it were me that did the $50 lending.
It would be different if my friend and I were hard up, and $50 were hard to come by. In that case I would ponder the $50K split, and like to think I would do it.