So, basically, going for Rachel that one time messed up things. DO you know in the save edit, which variant i can change to fix this?I don't want to be a wet blanket with these obvious things I'm about to say right now but you just need to strictly follow the guide and score relationship points with her whenever you need to. You first meet her at a freshmen party (Nymphomaniacs? Hell yeah! + 1 Nikki). If you replied on her message then you meet her at her studio for the first time and ask her about everything she's doing. Then at your first free roam you see her sitting on a suitcase and she offer you to help her at the studio (agree). Then she calls you, and you agree to come to her studio instead of helping Rachel (Yes). The second scene at the studio is a key one 'cause there you decide whether to help her and become her nude model for the exhibition or not to. If you agree, then at your second free roam you find her room door, her twin bro tells you about her last exhibition. Then you go to her studio, you don't find her there, you call her, a choice to ask her about the last exhibition appears during the conversation, and if you're developing the relationship with Roxy then you find a painting with Roxy naked in. Most likely, you just refused to help Nikki somewhere.
THe walkthrough gives you both options, and don´t mention consequences.