。
She accompanied me on the piano.
Literal
She [topic-は] me [to-に] piano's accompaniment [object-を] did [for me-てくれた].
伴奏 ('musical accompaniment,' literally 'companion-play') is the noun for an accompaniment in a musical performance — typically the piano or orchestra supporting a soloist. 伴奏をする ('to accompany') is the standard verb collocation. The configuration ピアノの伴奏 ('piano's accompaniment') uses the genitive の to mark which instrument is doing the accompanying. Note that ピアノを弾く would be 'play the piano' generically, while ピアノの伴奏 specifies the accompanist role.