Honestly, Felicity had expected almost any of the others to cheat (between Oliver's competitive side, her history of counting cards, and the time Cisco tried to cheat at Goldfish any one of the them could easily been accused of cheating when they decided to play poker) but, the last person she thought would cheat would be Barry — mostly cause he always just seemed too good a guy for that sort of thing.
So, it was to both her surprise and utter amusement when she went back to review the tapes (cause there was no way in hell her card counting was that off, she'd used that skill to pay the bills through high school dangit and even if she still managed to win last night, something had been fishy the entire game) and found that the Flash had been using his skills to quickly go through the deck and peak at other peoples hands; however, as Felicity kept watching the tape, she'd come to realize that Barry wasn't cheating for himself, he was stacking the cards in her favor (literally.)
Felicity wasn't quite sure what to do with this information since there was no way she was going to rat him out to the others (she'd lose her own killer card cred, after all) but, she didn't want Barry to think he was too fast for her to catch him; so, in the end, she decided to call and leave him a simple message, "Hey, Barry, don't let me forget to take you with me if I ever decide to go to Vegas."