(2-part post):
With only a few exceptions, someone can leave their estate to whoever they like and nobody else can do anything about it. Those exceptions generally only apply to persons who were in some way dependent upon the deceased. So, for example, a will which left nothing to a person's school-age children could (and almost certainly would) be contested. However, once those children reach an age where they are no longer dependents of the person writing the will, it would be perfectly in order if they were left nothing.
As there is nothing in your post to indicate that your friend's late father-in-law had any dependents, he was free to leave his money to any person or organisation of his choice. (He could, for example, have left it all to Battersea Dogs' Home. Nobody in the family, other than any dependents of the deceased, would have any right to contest this).
So, as long as the sister-in-law is being truthful when she states that she is the sole beneficiary of the will, she has every right to retain the full proceeds of the estate.