。
What a kind heart she has!
Literal
She [topic-は] what kind heart [genitive-の] possessor [explanatory-な] [I wonder/exclamation-のだろう].
なんと…のだろう is a fixed exclamatory frame meaning 'how X! / what an X!' — used to express admiration or surprise at the degree of something. なんと introduces the exclamation, and のだろう closes it with a wondering, contemplative tone (literally 'I wonder, isn't it the case'). 持ち主 ('possessor, holder') paired with the noun 心 ('heart, mind') gives a slightly formal, even literary 'one possessed of (such) a heart' — more polished than just 優しい人だ ('is a kind person').