I just beat the game again, and I realized that he didn't betray her. I thought she used a different door knob, so I thought Sai was trapping her. After I beat the game again, I realized that it was the return door knob that she used, so I guess he didn't really betray her. I don't know why I didn't realize it was the return door knob she used in my first playthrough, so that's my bad.