You know this has happened to me more than once. I really think that you don't owe someone you don't like and explaination or a gift. The best gifts are given from the heart and if yours is not in it then what's the point. I'm sure some people feel obligated to mix professional and personal relationships but I think that you have the right to choose who's life you want to participte in outside of work and who's you don't. If there is a formal invite involved, simply return the RSVP card stating that you will not be in attendance. I'm sure others who are participating my not be all that "chummy" with her either but are just playing the roll. My motto, always stay true to yourself, it limits the confusion. Good Luck1!