Flossie Wright had her reasons for choosing Sandy Cruz for the card game.
First and foremost, Sandy's family was wealthy; in other words, they had money, so he wouldn't care about the bit he'd lose in the card game.
Secondly, he didn't know how to play cards. He might be able to learn in a day, but he couldn't control the first few rounds, so he could lose money.
Lastly...
Though his demeanor was cold, he actually had a good temper, so he would cooperate with Flossie Wright to the greatest extent.
In fact, Flossie Wright had considered others, but she had ruled them all out.
Considering Rowdy Brightwill's personality, abilities, and his popularity among the female soldiers, he should have been the best choice, but lately, Flossie didn't want to get too close to him.
As for Becca Reyes and Belinda Nichols, one was too good at pretending, the other too serious; it was easy for them to give the game away, so Flossie Wright excluded them as well.
