As far as I know marijuana is not usually prescribed for anxiety. In fact, especially for folks who have never done it before it can cause severe panic and anxiety like one has never experience before and I am not sure that someone of her age could handle it very well. It may (and to be honest) probably would only compound the problem.
Has she been to a differnent doctor to get a second opinion? There are many medical condiditons that can cause anxiety/panic that can be relieved with the right treatment. I only ask because, while it is not unheard of, in all the research that I have done on the subject, I have found that at that age anxiety usually lessens for the simple fact that the chemistry that causes it is less active.
Also, are you (or is she anyway) religious at all? I would imagine that mortality anxiety would be significant at that age. It is not fun to talk about, but a reality all the same. There is much that can be said about the peace of mind and theraputic benefits of going to church. If she does not, I would suggest that as an option as well. Good luck.
Mike
I'm sure at 83, she would look a right picture smoking the weed. If it was my gran I would be concerned about her age going near anything not prescribed by a doctor. The body doesn't get stronger with age. So if it is really for gran I would be asking a doctor not a crowd of forum users. Because at her fine age it has to spot on.