When Naina came to know or Arjun being the real culprit, she wasn't as close to Sameer. Naturally, that time to her, telling Sameer would mean defaming her family in front of someone who's barely a friend. Though she should have told him that time itself, since he was the one who had suffered due to this fiasco, we can still forgive her, knowing her innate nature. In Mt.Abu, I think she didn't want to spoil all the good moments by mentioning something she knew would be in a bad taste to both of them. But later, had the bet issue not happened, I'm sure she would have told him. Maybe that day itself! Sameer was upset because of his mother that time, and Naina was there to comfort him. Maybe while talking about his misunderstandings with her mother, while that cheating issue came up, she would have told him. Now of course, she considers him as being a cheater, so that issue pales in comparison.