。
I've met her somewhere before.
Literal
She [topic-は] somewhere [at-で] met experience [subject-が] exists.
彼女 fronts as topic with は, but the subject of the meeting verb is the unmentioned speaker — Japanese is happy to drop the doer when context fills it in. A more canonical phrasing would slip に in before 会う to mark the person met, but topicalization with は absorbs that role here. ~たことがある supplies the 'have experience of doing' frame.