。
She got absolutely furious.
Literal
She [topic-は] blazingly got-angry.
かんかん is a 擬態語 (state mimetic) for blazing intensity — the sun beating down (太陽がかんかんに照る), an iron grill glowing red, or as here, someone seething with anger. The に attaches the mimetic adverbially to the verb 怒る ('get angry'), giving 'angry to the point of blazing.' Note the past form 怒った: 怒る is one of those verbs where the simple past form often describes an immediate state ('she got angry'), since the verb itself names the moment of becoming angry rather than the ongoing condition.