Some of the ways to make her feel better is by watching funny movie to make her happy or do some exercises together. However, you may also tell her that an argument might not mean anything apart from that one parent or both just lost their temper.
My parents are same. But none of my two exes had enough sense to make me feel better about that, instead they were always complaining that why i care and i should chill ! Freaks ! So please, never ever say her to forget everything and enjoy. It'd seem that you are insensitive. Just tell her that everything will be allright and whatever happens you'll be with her forever and she can trust you with anything. Whenever she shares her parents matter with you, never be too critical and instead of loading her with advises just listen to her. Take her out for a movie or just hang out together or with some friends. Just divert her mind.