My step dad did lose his rag once when I was being extremely rude to my Mum. Never caused any issues, I was being an idiot To be fair, I think it is within your remit to say it. If your partner was being treated like that by a stranger ,wouldn't you say something?
Over and above that, her Dad is not the only significant male figure in her life; it's not really about rights, you have the opportunity to show her a bit about how to treat other people. If her Dad is a waste of space, then you should be a more positive role model.