Totally agree with the fact that context is needed. Even with Jane or Cassie I would act totally different. With Jane I might take a subtle approach about trying to very stealthy show her that her boyfriend is a douche. With Cassie, I might be way more dominant and direct. Not just because I think it will backfire with Jane, because MC relationship with Cassie is that of a father and daughter, the level of allowed intromission into her life can be assumed to be bigger.
Wait, did I write father and daughter? I meant neighbors..
Hehe, I'm personally indifferent to cheating content. So, banging the GF while she is basically cucking her BF isn't really interesting to me. But I definitely have a bit of a white-knight complex, so I'd be far more interested to help her get away from him and or protect her from the BF.
To expand on the scenario I posted originally.
This ex-boyfriend douche is obsessive. He starts to cyber-stalk Jane/Cassie. MC notices that Jane/Cassie are distracted, by something. MC asks them about it. They reveal their ex is trying to get back with them.
Red Jane is interested. She and her ex did a lot of crazy stuff together and its sort of intriguing to her to go back.
Blue Jane isn't interested at all. She doesn't want to go back to that time of her life. She is trying to change herself for the better.
Red Cassie wants to do it. The crazy ex is actually her first boyfriend. Her first kiss, took her virginity. She can't see that he is a bad influence on her.
Blue Cassie is scared. She broke up with him. She wasn't happy with the relationship and is now afraid of him because he is a bad guy.
But the ex doesn't care about any of that. He just wants some pussy. Pushy, arrogant and a massive douche to say the least. MC must step in and save the day!!
For the sake of not having too many branches, the ex isn't successful on any route.