。
She treats him harshly.
Literal
She [topic-は] he [to-に] harshly behaves.
つらくあたる is a set phrase using 当たる in its less common sense of 'to behave toward (a person)' — に marks the recipient of the behavior. つらく is the adverbial form of the i-adjective つらい ('painful, harsh'). The plain present tense here describes habitual or characteristic behavior, not a one-time event.