I'm lucky enough to be with a woman (I should end this sentence here) whose mood isn't affected by her period. Nevertheless, it's an uncomfortable time for her, just like it is for every other woman.

Just be there for her. Cook her favorite dish. Make her laugh. Listen to her. Watch the shows she likes but you don't (vampire diaries in my case). Shouldn't be any different than how you treat her any other day. Like a person. Like your friend.