It happened to me. My boyfriend's mom hates me, still does. Sit down with your girlfriend and tell her that you do not like how her dad is being with you. If you're too shy to tell him yourself (lot of people are) just doi thath, or get some courage and tell him yourself :) be respectful!!! Sit down and talk with im, thatll do the ytrick
The best way to deal with your girlfriend's dad is by being nice and friendly to him . Never lose your cool in front of him or show any form of disrespect. Listen to whatever he has to say but block the hurtful words out.