How old are you and how old is your girlfriend? If you are both consenting adults there is not much her father can do to force you apart. It is possible though, if she lives with him, that he can make her life miserable if she continues to see you. Write her a note telling her how you feel and asking her if she is going to obey her father or not. If she says she is, there is not much you can do. If she says she is not, she will have to pay the consequences and they may not be pleasant.