The naive approach would be Ian just helping Cindy out as a friend and falling for her involuntarily the more the two hang out together and the more its apparant that her relationship with Wade is doomed. If she would break up asap it may not even be the worst betrayal ever, since people can end relationship and start new ones without being scumbags. But the way Ian and Cindy hesitate, let it a secret and priorize on their lust at the expense of the rest of their friends, its assholish.
Then there is Chad Ian who is just fucking her as a sidepiece.