Here is what I would have done if I were any of her friends or the other witnesses

If I were any of Stacy's friends or the other witnesses and was gonna report Bobby's abuse towards her to the police, I would have gotten in touch with the girl who was with Bobby before Stacy and asked her to confront Stacy and force her to admit that Bobby hits her and it might not have been easy and it might not have been fun but it might have prevented Stacy from getting killed.