Am I wrong for leaving a message on my son's girlfriends cell phone tell her not to hurt him physically.

My son has been dating this girl for 1 year. Since she came in his life he's been doing poorly in school, slashed a tire on an ex gf's car because new gf asked him to, to prove his love to her. Now she is physically abusing him. Scratching, biting, hitting, pinching, destroying his property. His face was scratched up this past thursday. I reached my limit. I called the girl and left a message on her cell and said that she is to leave my son alone or I will call the cops. Now my son ran back to her and wont talk to me saying I was wrong to say that.