。
She regretted not having listened to his advice.
Literal
She [topic-は] his [genitive-の] advice [object-を] not-listened-to fact [object-を] regretted.
A nominalized embedded clause as the object of 後悔する ('regret'): ~なかったことを後悔する ('regret the fact of not having done'). ことを turns the embedded past-negative clause into a noun phrase. きく here means 'listen to, heed' (the figurative sense, often written in kana), with the heeded thing taking を — distinct from ~に従う ('follow, obey'), which would take に. 忠告 ('advice, counsel') is heavier than 助言 — implies advice meant to warn or correct.