When she comes back she find out he was awake and he asked her where she go,then she did told him that she got to Damien and fucked him and she asked "are you upset about it" and you has the two options "no,i can never be upset with you" and "yes,i am a bit upset" ,if you don't remember it i suggest you play it again and see by yourself - that's no longer cheating since she did tell him,if she did lie to him then it was gonna stay cheating but she didn't lie him so is no longer cheating,just the beginning of making him a cuck.