Sensei seems to know he's holding on to pieces that will keep Noriko around him, and she's basically lived her life trying to be his dream girl. She could move on, but I don't really see it happening.
Niki thought Sensei might still be a virgin since they never had sex:
So, I doubt she was aware of him having that relationship with Sekai.
Her letters in the Chapter 2 'Spotless Mind' Main Event, seem to imply she wasn't sure why he ghosted her.
Although, I'm beginning to wonder if the accident or Sekai was even the actual reason he did. Niki seemed to remember Maya and Maya was around within months of Sensei ghosting Niki. Maya simply may have not wanted to share.