。
She's supposed to come visit me this afternoon.
Literal
She [topic-は] today 's afternoon me [object-を] visit-and-come [established-arrangement-ことになっている].
~ことになっている marks an established arrangement, schedule, or rule — something agreed upon, not a fresh personal decision. It's the natural way to describe appointments, work shifts, club rules, or social customs. Compare ~ことにする ('I (have decided to) do X'), which centers the speaker's choice, with ~ことになる ('it has come to pass that X'), which centers the resulting state. The compound ~てくる layers motion ('come') onto the main verb 訪ねる, presenting the visit from the speaker's perspective: she will move toward me.