。
What a chatterbox she is!
Literal
She [topic-は] what chatty [I wonder-なんだろう].
おしゃべり is one of those Japanese words that doubles as a noun and na-adjective — 'chatter,' 'a chatty person,' or 'chatty' depending on context. Here it functions as an adjectival noun. The ending なんだろう combines the explanatory ん + plain conjectural だろう, giving a slightly reflective 'I wonder how chatty she is!' flavor — closer to musing aloud than addressing anyone. 何て~なんだろう is a stock pattern for half-amazed exclamations.