Complex sentence: When she returned home, she replaced her pianist-a good friend of Marian's-and began working with a Finnish accompanist, Kosti Vehanen-a critical move for Marian's career, to work professionally with a white man in the United States.
Split into simpler sentences: When she returned home she replaced her pianist--a good friend of Marian's--and began working with a Finnish accompanist, Kosti Vehanen. This was a critical move for Marian's career, to work professionally with a white man in the United States.