。
How kind she is!
Literal
She [topic-は] what kind [explanatory-な] [I wonder/exclamation-のだろう].
親切 is a na-adjective, so the explanatory nominaliser appears as な — 親切なのだろう, parallel to noun-predicate なのでしょう. The structural pattern なんて+[adjective]+のだろう (or のでしょう for politeness) is one of Japanese's most flexible exclamation frames, working with na-adjectives, i-adjectives, and verbs alike. The plain だろう version (vs. polite でしょう) lands here as more reflective or to-oneself — the kind of thought you'd whisper rather than declaim.