It's great that your mother named you as executor, since this is usually an indication that she trusts your judgment or business savvy.
There is no reason for an attorney to update a will if nothing is changed. I assume that this attorney or another one has helped her draw up this will.
On the one hand, your mother's will is a private document that, if she decides it should remain confidential, then you should respect her wishes.
However, in this case, the evidence would seem that some type of change was done on the will which may or may not have important consequences.
You should explain to your mother that as executor, you are going to be the person who is responsible for carrying out her wishes and it is better for the executor to have some idea ahead of time how the estate is to be divided so that you can carry out the probate process in the most efficient manner when the time comes. She may also want to consider having a second attorney review the will to make sure that all legal language is phrased properly and correctly--if there are any mistakes in the language that could cause a double meaning, etc. the mistakes need to be discovered and corrected while she is alive--because it will be too late to correct it after she is gone, and her monies/properties may end up being handled in a different manner than she intended. Someone must also check to see that the correct number of witnesses have been used and her signature should be notarized.
You must promise her that you will keep the contents confidential (and then do just exactly that), and tell her also that you need to know where the original will is being kept so that you can retrieve it after her death. At least 2 people in the family besides the attorney should know where the will is kept, just in case the attorney goes out of business or for some reason could not be located at the time of your mother's death.
Do not blame or get mad at the attorney for whatever advice he has given your mother, as he simply is respecting her desire for confidentiality, and you may have to work with him cooperatively if he is involved in handling the estate after your mother dies.