。
How beautiful she is!
Literal
She [topic-は] what beautiful [explanatory-の] [I wonder/exclamation-でしょう].
なんて is the colloquial sibling of なんと — both introduce exclamatory 'how / what a' phrases, but なんて leans more conversational and emotional, なんと more written/composed. Combined with the polite でしょう (instead of plain だろう), this lands as a refined, slightly old-fashioned exclamation, the kind that might appear in a romance novel or formal narration rather than spontaneous everyday speech.