Update: John Thayer Jr. died on September 20, 1945, age 50.
That was 17 months after Marian died, at age 71.
For Marian Thayer, her death was probably due to natural causes.
The obituary, for John Jr. (posted on the Passenger Information site) mentions the death of Mrs. Thayer:
"'....he had redoubled his efforts in the war period, especially after the death of his son, Edward, in the Pacific, which was followed closely by the death of his mother.' Mr. Thayer's mother, Mrs. Marian Longstreth Morris Thayer, died at her Haverford home April 14, 1944, which was the 32nd anniversary of her husband's death on the liner Titanic, which sank after striking an iceberg in the Atlantic....".
Marleen, what you might want to research was the closeness of mother and son, in the years after the tragedy. How did Marian Thayer's death affect her son? This may make an interesting aspect for your project.