The thing is. She never moved on. Not one bit. Takt only made a move on her. When she mentioned Rikus name, and they were friends. Takt is the one who took advantage of her. Then dropped her. He even stated why. I think he moved on to what's her name only because I think Riku was planning to confess, but Takt being the POS that he's he did something to force What's her name into dating him. In the backstory you can see something isn't right. He said, and did something. This is a good example of being careful. Can you really trust your friends.