[Marsh.]

Regardless of the amount of time they've been seeing one another, if they'd reached the stage they were actually referring to one another as boyfriend/girlfriend or whatever then they entered a relationship and she is cheating.

When asked, Sarah might make it sound more like a casual friends with benefits type situation which is what it actually sounds like to me. She seems the type to rush into "omg I love you, let's get married" and then have a complete breakdown when it doesn't work out.