How do I help my 16 year old daughter deal with a grown man bullying her by spreading false rumors of a sexual nature?

Report all evidence, including texts or social media to the police and the local domestic violence shelter hotline for legal services and an injunction. Try to seek therapy for her to deal with the emotional aspect of this. And also remember that this may not "pass" even if you live in large area, depending on how it is dealt with now, so again, please try to get free legal assistance, talk with a local domestic violence shelter about getting a referral for free legal aide. and try to get yourself some help, too, mom, because you will need it to get through this, it sounds like your being put through something horrendouse for no reason