We haven't found the death certificate for Theodore Prather in Minnesota Historical Society archives online. The records start in 1908 (with a few included from 1904-1907). Therefore we can guess he died before 1908. The historical society would have his death card on microfilm and available on site in Saint Paul. Maybe write to them to ask for a copy.
You do now have Martha Prather's death date and place (10 August 1932 in Hennepin County). Her Minnesota death certificate # 1932-MN-020437 can DEFINITELY be ordered for a modest fee and should mention her burial place. Once you have that, you can write directly to the cemetery to ask whether Theodore is buried with her.
Some facts you may have, or maybe they will be helpful:
1940 Census Minneapolis, Hennepin, Minnesota 1501 LaSalle Avenue Eugene Harrison Prather 48, divorced, lodger, b abt 1892 Minnesota, outdoor advertising …this seems to be an apartment house or large boarding house…other people here… Residence in 1935: Same Place (a question on the 1940 census)
1940 Minneapolis, Hennepin, Minnesota 1824 5th Street North Andiee Amundson 42, head, b about 1898 in France, divorced Elmer Amundson, 20, son, b about 1920 in Minnesota Raymond Amundson, 17, b about 1923 in Minnesota Walter Prather 44, widowed, lodger, b abt 1896 in Minnesota Walter’s Residence in 1935: Same House