。
She was kind to me, you know.
Literal
She [topic-は] kindly did [for me-てくれた] [you know-よ].
親切にする ('to be/act kind') treats kindness as something one does — the na-adjective 親切 in its adverbial form 親切に combines with する to form a behavioral verb, parallel to 静かにする ('be quiet') or 大切にする ('treat as valuable'). The auxiliary ~てくれた then re-frames the act as something done for the speaker. Sentence-final よ marks the speaker's confidence in the information and a hint of insistence — 'just so you know,' often used when correcting a mistaken impression about someone. The implication is something like 'she was good to me, despite what you might think.'