When Marilyn and Joe got divorced in 1955, Marilyn did not state to the court she was divorcing on grounds of physical cruelty.
Neither Marilyn nor Joe ever discussed this in any letters, and Arthur Miller (her third husband) never mentioned it either. Marilyn was also very open about the abuse she faced as a child, and she confirmed that she left Joe on grounds of "mental cruelty" but they still saw each other until mid-1955.
After her divorce from Arthur Miller, their friendship was rekindled.
You can read more about their relationship here