Colloquially, they are often used interchangeably.
The technical difference is the verb tense and what that implies.
“She has used it” is the Present Perfect tense. The action happened in the past but is relevant to the present. It emphasizes a completed action.
“She has been using it” is the Present Perfect Continuous tense, where the action started in the past and has continued up until now. It emphasizes the ongoing action.
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