NEVER, EVER threaten him. Document EVERYTHING he does that she finds terrorizing.
And if he should happen to be in a biker bar and lose an incisor or three, make sure the message gets to him that he needs to learn to be a gentleman and treat your daughter with respect if not totally back off...
It might cost you some PI (private investigator) time and effort.
:-)
All I told him was I was off parole. ;-))