Frankly, without a written agreement to pay you back; I think you are screwed. Remember Bill Shakespears advice: Neither a borrower, nor a lender be. You *could* talk to a lawyer: see what he says. You *could* threaten to tell his wife: see how that goes.
Me? I would be busting knees with a baseball bat!
Of course, I never EVER would have lent such a sum, unless I could afford to lose it!
And to a degenerate gambler, no less! ......screwed!

