How can I get an elderly guatemalan citizen an extended visit to the U.S.?

My best friend and I are trying to work out having his grandmother come to the U.S. for an extended visit from Guatemala. Of course it's hard to prove that she has strong "reasons to return", as she is no longer working. I had heard however, that it was possible for me to write a letter or something? (My best friend only has his greencard and has not yet taken his citizenship test) How do I go about helping her come visit?