I think this is where it will start. The girlfriend will start to cheat if Nanase continues to act like this. There's also a chance that she may fall in love with the girl she's cheating with but that would ruin the fun, right? That would be so predictable.
So I'm guessing that the girlfriend will remain in love with Nanase and keep things cool with her, but she will also continue cheating on her with the long-haired one just so she can cover the emptiness she feels when Nanase is forgetting every little yet important thing abt their relationship.
But I honestly think that no matter what happens, she will always choose Nanase. The long-haired girl agreed to be number two, so I guess she's all good with this. Unless the main protag gets her feelings tangled up and becomes confused. She might even leave Nanase for good because the girl is too busy and our protag is not too patient.