Avoidance is good short term, but, a friendship wherein each is able to be emotionally honest (of course NOT when she's PMSing) about this kinda stuff is my goal. Plan during the good days for the bad days. Set up code phrases to bring up when she's going down. "Honey Baby Sweetie Pie, it's time for my monthly time out. Have a lovely period. Call me when it's over" type triip. You both get to be on the same page and claim membership in a mature friendship. Nothing to sneeze at, eh? Takes 2 to tango. Good Luck.