。
She bore the sorrow well.
Literal
[topic-は] well sadness [in-に] endured.
よく here means 'well, admirably' rather than 'often' — context disambiguates: with a single past-tense verb of effort or virtue, よく functions as a praising adverb meaning 'admirably, despite the difficulty.' Compare よく頑張った ('you did well, kudos for trying'). 悲しみ is the noun form of 悲しい, used here as an abstract object endured. ~に耐える ('endure X') marks what is being endured with に — the sorrow is the thing borne up under.