I am a reader of Priscilla's column and I am writing to give some suggestions about your problem with your friend.I know what it's like to feel angry and confused because your best friend has been stealing for months.
A good idea to prevent this might be to speak to her parents, in order to know which their daugher's problem is.I think that parents are the closest people to each one of us; they are the people who have only positive thoughts about their childen and the people who know better than anyone the weak points of them. In this way, parents are the most suitable people for such a serious problem and they have the right to know about it.
Another good idea is to speak to a person who you know that your best friend respects, trusts, and appreciates. For example, a teacher, a relative, or even better a psychologist. In my opinion, this person can approach your friend because she will feel that she speaks with a friend and she won't defent herself in what she\he has to say about her problem.It is urgent that somebody should made your friend understand what is happening to her and the most important is to find out the reasons why she is in this difficult situation.
Taking all the above into consideration, I think that you should help your best friend and, thus you will express your feelinds and your concern about her. I am sure everything will be fine in the end.