。
She was unkind to him.
Literal
She [topic-は] him [to-に] unkind was.
不親切 ('unkind') uses the productive 不- ('not, lacking') prefix on 親切 ('kind'). The に here marks the target/recipient of the unkindness — '(toward) him.' 不親切だ as a na-adjective + copula expresses the state. Compare with 不親切な人 ('an unkind person') as a prenominal modifier.