Question: Who did David McCullough mention in The Greater Journey that befriended Edgar Degas in France?

Context paragraphs:
A Cotton Office in New Orleans: A Cotton Office in New Orleans is an 1873 oil painting by Edgar Degas.  In it, Degas depicts his uncle Michel Musson's cotton brokerage business (which several years later went bankrupt in an economic crash, according to Michael McMahon of the "Pittsburgh Post-Gazette" when the firm was swamped by the postwar growth of the much larger Cotton Exchange).  In the painting, Musson is seen examining raw cotton for its quality while Degas' brother Rene reads "The Daily Picayune".  Another brother, Achille, rests against a window wall at left while others, including Musson's partners, go about their business.

Waiting (Degas): Waiting is a pastel on paper by the French Impressionist Edgar Degas, completed between 1880–82.  It is an early example of the more than 200 pastels, paintings, mixed media drawings and sculptures of ballerinas depicted by Degas from the early 1880s.  This work is regarded for its vibrant colouring and steep perspective.

The Greater Journey: The Greater Journey: Americans in Paris is a 2011 non-fiction book by the Pulitzer Prize-winning author David McCullough.  In a departure from McCullough's most recent works, Founding Fathers like Benjamin Franklin and Thomas Jefferson, who spent time in Paris, are not covered.  Instead, the book is about 19th-century Americans like James Fenimore Cooper and Samuel Morse, who migrated to Paris and went on to achieve importance in culture or innovation.  Other subjects include Elihu Washburne, the American ambassador to France during the Franco-Prussian War, Elizabeth Blackwell, the first female doctor in the United States, and American artists who worked in Paris such as George Healy, Mary Cassatt, and Augustus Saint-Gaudens.

Degas (crater): Degas is a rayed crater on Mercury at latitude 37.5, longitude 127.  It is 45 km (27 mi) wide and was named after Edgar Degas.  The rays consist of light colored material blasted out during the crater's formation.  Craters older than Degas are covered by the ray material, while younger craters are seen superimposed on the rays.

The Ballet Class (Degas, Musée d'Orsay): The Ballet Class (French: La Classe de danse) is a painting by Edgar Degas, which was painted between 1871 and 1874.  It is in the collection of the Musée d'Orsay, Paris, France.  It was commissioned by Jean-Baptiste Faure.  Degas temporarily abandoned work on this painting, and delivered a work of a similar name to Faure.

The Millinery Shop: The Millinery Shop (1879/86) is a painting by French artist Edgar Degas.  It depicts a woman sitting at a display table in a millinery shop, appearing to closely examine or work on a lady's hat, which she holds in her hands.  The view of the scene is at an angle from above.  Although Degas created several paintings concerning milliners, this painting is his "largest and only 'museum scale work' on this subject".

The Dancing Class: The Dancing Class is an oil painting by Edgar Degas.  It was painted about 1870.  It was the first of Degas's "ballet pictures".  The painting depicts a dancing class at the Paris Opéra.  The dancer in the center is Joséphine Gaujelin (or Gozelin).

Mary Cassatt: Mary Stevenson Cassatt ( ; May 22, 1844June 14, 1926) was an American painter and printmaker.  She was born in Pennsylvania, but lived much of her adult life in France, where she first befriended Edgar Degas and later exhibited among the Impressionists.  Cassatt often created images of the social and private lives of women, with particular emphasis on the intimate bonds between mothers and children.

The Suites, New York 1968 &amp; 1970: The Suites, New York 1968 & 1970 is the fifth volume of "The Private Collection", a series documenting recordings made by American pianist, composer and bandleader Duke Ellington for his personal collection which was first released on the LMR label in 1987 and later on the Saja label.  The album includes "The Degas Suite" which was commissioned for an uncompleted film and inspired by Edgar Degas' paintings of racetrack scenes.  Also featured is "The River" which was composed to accompany a ballet by Alvin Ailey which premiered on June 25, 1970 at Lincoln Center's New York State Theater.

Interior (Degas): Interior (French: "Intérieur" ), also known as The Rape (French: "Le Viol" ), is an oil painting on canvas by Edgar Degas (1834–1917), painted in 1868–1869.  Described as "the most puzzling of Degas's major works", it depicts a tense confrontation by lamplight between a man and a partially undressed woman.  The theatrical character of the scene has led art historians to seek a literary source for the composition, but none of the sources proposed has met with universal acceptance.  Even the painting's title is uncertain; acquaintances of the artist referred to it either as "Le Viol" or "Intérieur", and it was under the latter title that Degas exhibited it for the first time in 1905.  The painting is housed in the Philadelphia Museum of Art.

Supporting paragraphs: The Greater Journey Mary Cassatt
Answer: Mary Cassatt