。
She was very kind to me.
Literal
[topic-は] I [to-に] very kindly did-for-me.
よくする ('to treat well') is a set expression — the adverb よく ('well') plus する ('to do') — that means 'to be good/kind to someone.' Adding the giving auxiliary ~てくれる shifts the perspective onto the speaker as the beneficiary of her kindness, a Japanese-specific bit of grammatical bookkeeping that English flattens into 'she was kind to me.' The に marks the recipient of her good treatment. This 授受表現 ('giving/receiving') system encodes who benefits from an action — a pervasive feature of Japanese that learners sometimes underuse.