Beat her up? I think I would let her go her own way. Things sound kind of out of control right now if your Mom is threatening to beat someone.
And why does your mom believe the 3rd party info over what you've told her? Is this information true? If the information is true, will you be harmed by having a relationship with this girl?
Try to see why your Mom is upset. Most parents want to protect their kids and look out for their best interests. If you can show your Mom with facts that this relationship is good for you, maybe she'll calm down.
