。
She's visiting Paris.
Literal
She [topic-は] Paris [object-を] is-visiting.
訪れる ('to visit') is a slightly literary alternative to 訪ねる, more common in written prose and polished speech. It also has an extended sense of 'arrive, come over' for seasons or moods (春が訪れる 'spring arrives'). The progressive 訪れている describes the ongoing-state of her visit — she's currently there. を marks Paris as the place visited, the standard pattern for 訪れる/訪ねる, distinct from に for static location.