I don't think you should not go just because it is a Mary Kay party. I have had plenty of friends in the past that have had Mary Kay, Tupperware, Pampered Chef, etc., other type sales parties for the excuse to get together. No one felt they had to buy, and it was usually fun. Remember, without holidays and birthdays, etc., some people feel there is no excuse to have get togethers.
Call your friend and ask her if there will be JWs there and let her know that you don't want it to be ackward for you or her. If you really considered this person a friend, I say go. If she studied long enough to know how JWs are supposed to shun people, then she is being gutsy inviting you because she misses you and doesn't want to shun you.
If you don't want to go, just call her and ask if you can get together for coffee or something. It sounds like she misses you too. I would give her the benefit of doubt.