If the person plays poker and believes in God this is by no means a sin because you have no participation with any religion, now if you are of any religion and play poker maybe it can be, because some do not accept games, drinks and etc ...
playing poker is definetly not a sin.. its fun i dont even think its gambling because its more a game of skill and gambling is playing the lotto or slot machines where its all luck and your hoping to win.. even tho people on these sites play like there in a bingo tournement and go all in pre flop every hand just to see how lucky they can get.. but yeah i dont think its a sin at all