The should have made Rose's boyfriend be dead from earlier.

Rose's main driving force in the movie is the feel of guilt for how she treated her previous boyfriend who died after a fight. Well, we know she is exaggerating her guilt, but *she* is doing it nonetheless. A person like that wouldn't start dating another guy in few days. If you feel bad about hurting someone, you aren't going to 'move on' right away. Also, the casual reaction of his parents to seeing her dating another guy would be much more understandable if some time has passed. There actually isn't a single thing in this story making it necessary for him to be recently deceased. It just makes Rose look too hasty and not overtly ridden with guilt, which was the point.