。
She's a complete stranger to me now.
Literal
She [topic-は] now [as for-では] me [to-に] [contrast-は] complete 's stranger is.
他人 ('other person, stranger, outsider') is the specific Japanese term for someone outside one's circle — the opposite of 身内 ('one's own people'). This in/out distinction is a cultural backbone: 他人 doesn't just mean 'unknown person' but 'someone outside the trust circle,' a category that can include former friends, distant relatives, or anyone now relationally remote. 全くの ('complete, utter') intensifies the noun directly via の, a productive pattern: 全くの嘘 ('a complete lie'), 全くの初心者 ('a complete beginner'). The doubled には marks 'to me' with a flicker of contrast — perhaps not to others.