。
She was angry at the terrible way she'd been treated.
Literal
She [topic-は] terrible treatment [object-を] received [quotative-と] was-angry.
扱いを受ける ('to receive treatment') is a slightly formal idiom built from the noun 扱い ('treatment, handling') and the receiving verb 受ける. It's the impersonal way to talk about how someone was treated — neutral about who the agent was. The と after 受けた quotes the content of her anger: she was angry over the proposition 'I received bad treatment.' 怒る in the past progressive (怒っていた) describes a sustained emotional state rather than a single outburst, the standard pattern for emotion-verbs in narrative.