It's up to the player. You can either decide for one of them (or both) to break up, or to stop (or keep) cheating. If Ian is informed by Jeremy that Lena has been cheating on him, he can choose to confront her about it or stay quiet. If you confront Lena, you have option to turn their relationship into fully open arrangement where they both can fuck other people, but stay together. Alternatively, if Ian got over Gillian he can also simply forgive Lena.