。
She thanked us for our help.
Literal
She [topic-は] our help [for-に] was-grateful.
感謝する ('to be grateful') is a formal suru-verb pairing with に to mark what one is grateful for. 手助け ('help') is a native Japanese compound, slightly less formal than 援助. The plural 僕たち folds the masculine first-person pronoun 僕 with the plural suffix ~たち.