Any idea why she has gone cold? Can you ask around in your group of friends to see it you can find out what it may be? If it's nothing other than a personality clash and if you have always done things as a group of 5, I think I would invite her - it's up to her to make her excuses if she doesn't want to be a part of your event. If she does attend then there are 5 of you and that means plenty of different people to talk to and things should pretty amicable.