Since she was NTR'd by my senior, I'll NTR his girlfriend (WN)

First-year university student Yu Isshiki has a cute girlfriend, Karen Mitsumoto.
But one day, Yu discovers Karen is cheating on him with his senior, Tetsuya Kamakura.
Overcome with rage, grief, and despair, he calls Sakurajima Toko—
Kamakura's girlfriend and the “campus beauty queen.”
“Toko-senpai. Please cheat on me.”

But Toko replied,
“Simply cheating isn't enough. We must push him into the depths of despair.”
Her plan was
“To dump him when he's most infatuated, then spend the night with another man right in front of him.”