Start with the information you have on your grandmother.
I found a record that Thelma S. Heise was divorced from Michael J. Arthofer in September 1970 in Los Angeles. From this document you will obtain their date and place of marriage. Following the records, her application for the Arthofer marriage should tell you about the previous marriage and divorce.
I found other marriages for Thelma, so you will have to follow the paper trail. You seem to have most of this information in your family tree.