Changing a will I made with a friend in 1984?

In the event of my death, I bequethed all my money to my friend and she did the same, but when we have both passed away everything was to go to my Sister, who was the executor. By chance I came accross a new will dated 11/05/2011. It is my friends will in which she is leaving all her money to me, so long as I survive her by one month, if I do not then all the money is to go to Charity, hers and mine, I do not whant my money to go to Charity! I s she allowed to do this, I would not have known if I had not come accross her new will.