“Boyfriend” is being used to clarify the nature of their relationship - specifically that this person was not a family member but came from outside the family, and that their relationship was sexual as opposed to a non-sexual.
If the headline just said “abuser” then it’s less informative and more ambiguous how the victim and perpetrators knew each other. “Abuser” is so broad that it doesn’t tell you anything about how the two people know each other, and could include family, sexual contacts, friends/aquaintances, or even a bully.
I think it’s just a shorter way to say “so-called” boyfriend. Obviously any sane human would see this is wrong and not a healthy situation, but why leave it up to the reader to make that distinction?