Stephanie, I am so sorry you are having to deal with this. I saw a documentary a few months ago about women who communicate with incarcerated men and it was pretty scary.
I know "stuff" is nothing compared to your mothers well being but, if her mind is made up & she does marry him, she should at least protect herself financially for her future since she barely knows him. If she owns a house, I would STRONGLY suggest she look into at least getting a Pre-Nup to protect her home at the very least, and to protect her from future Alimony payments if they were to divorce. As I'm sure it would be hard for him to find a well paying job.
Again, I'm so sorry this is happening. I will keep her and your family in my prayers!