。
She left her son alone in the car.
Literal
She [topic-は] son [object-を] car [genitive-の] inside [in-に] one-person [adverbial-に] put-and-left.
~ておく ('do in advance / leave in a state') is doing important work here: 一人にしておく literally means 'make him alone and leave it that way' — 一人にする ('make [him] alone') + ておく ('leave that state in place'). The implication is that the state was left for some duration, often deliberately. 一人に here is the [na-adjective/noun]+に of ~にする ('make into X'). The whole construction speaks to a fraught real-world topic — leaving a child unattended in a car carries the same critical undertones in Japan as elsewhere.