They are both correct in the appropriate context. Your issue may be that you are not confident generally of when you should use the present perfect tense and when you should use the past simple.

Hello Andrea, If you are talking in the present and she is still with you, you should use 'She has never left me'. If you are talking about events in the past (and she is possibly not with you now), then you should use 'She never left me.' Hope this helps Bob

It depends on the context. You have to provide more context. Your question is similar to asking: Which is correct: "She laughed" or "she laughs"?