Yes, your friend was rude. When she was invited to the cookout, she should have told those people, "I'm sorry, but I won't be able to attend." Unless there was an emergency or other urgent matter, she should have honored her prior obligation she made with you. If you do not wish to spend time with her anymore, just simply tell her the same thing she should have said: "Thank you for the invitation, but I will not be able to attend. I apologize." Never lie and never over explain your reasoning, in fact, you are not obligated to provide a reason for turning down an invitation, so do not even try to offer one. Do not do or say anything that would lead her to believe that you would be willing to meet with her on a later date such as, "Maybe next time," "Let me check my calendar, and I will get back with you," etc. Simply thank her for the invitation, let her know you will not be able to attend, make your apologies, and leave it at that. If she asks you later on down the road, repeat. Most people will eventually get the message. You are not obligated to have personal relationships or meetings with your coworkers, so do not feel like you owe her anything outside of a professional, cordial working relationship. In fact, I always think it wise to never form personal friendships or relationships with coworkers in order to avoid these sticky, awkward situations.