If Ian is sleeping with somebody else besides Cindy ( for instance Lena) that Cindy is aware of. Then she's far more likely to succumb to Axel's charms, and Ian really only has himself to blame. If Ian is exclusive to her, then I don't think anything will happen between Axel and Cindy. And the reason for that, is because Cindy isn't a protagonist, the player can't affect her actions, only Ian's. So you have to make choices for Ian , that causes Cindy to cheat on him. So as long as Ian is exclusive to her, she can't really cheat on him, because otherwise they'd never end up together. But that doesn't stop Ian from getting insecure and believing that it might be happening though.