。
How beautifully she plays the piano!
Literal
She [topic-は] what skilfully piano [object-を] play [explanatory-の] [I wonder/exclamation-だろう].
上手に as the adverbial of the na-adjective 上手 modifies 弾く ('play [a stringed/keyed instrument]'); the same verb that covers violin also serves piano, since piano is keyed. The whole verb phrase 上手にピアノを弾く is then nominalised by の and exclaimed via なんて…だろう. ピアノ is a straightforward English-origin loanword (originally Italian, but borrowed into Japanese via English), naturalised completely.