。
She's attentive to how the customers are handled.
Literal
She [topic-は] customer [possessive-の] handling [to-に] attention [object-を] is-distributing.
気を配る is one of Japanese's many 気 idioms — 'to allocate one's 気,' meaning to be considerate, attentive, alert to what someone might need. The verb 配る ('distribute, hand out') gives the metaphor of spreading awareness around. 客の対応 ('customer handling, customer service') uses the noun 対応 ('response, handling, dealing with') in its applied sense, especially common in business and service contexts. The に marks the focus of the attentiveness. The ~ている here gives ongoing-state reading: not a one-off act but her habitual approach.