。
She accompanied the singer on the piano.
Literal
She [topic-は] piano [with-で] singer [of-の] accompaniment [object-を] did.
伴奏をする ('to accompany, provide accompaniment') is the standard light-verb expression for musical accompaniment. The の linking 歌手 to 伴奏 frames the singer as the recipient or focus of the accompaniment ('the singer's accompaniment'). で marks the piano as the instrument used. 歌手 ('singer') is the everyday word for vocal performers, broader than the more poetic or specific 歌い手 ('singer, vocalist').