Your girlfriend is an adult and does know how to live her life. Her parents shouldn’t blame you for her actions. They should talk to their daughter if they think something is wrong. She can’t be influenced by you. But just to convince her parents that you are not a bad influence, you can have a good communication with them. Assure them she is your girlfriend and you wouldn’t do anything which can influence her in a negative way. Try to prove them wrong. Make them understand that you are not influencing her in any negative way. They shouldn’t think badly about you. Try to change their perception of you.