You shouldn't talk shit behind someone's back.
Heather is a college student, and is friendly with a large number of other students including Dave and Ray.
Heather wants Ray to pay attention to her.
Heather tells Ray a story she heard about Dave helping out during campus clean up.
Heather tells Ray a story she heard about Dave, most of it unfavorable and only partly true.
Ray reacts favorably to the story, and suggests they get together with Dave later in the week for a study session.
Ray doesn't like how Heather is talking about her friend, and refuses to speak to her for the rest of the semester.