Do you think I should tell someone that my friend is sleeping with a teacher?
Do you have any evidence they are sleeping together?
I know she's insecure about herself around him, but when I confronted her about it, she denied everything
You should always report a teacher if they are sleeping with a student, no matter what. If it's not reported then this teacher could prey on other students.
I don't want to involve myself.
I understand that this is a difficult situation. Can you report him anonymously?