。
She left the window wide open.
Literal
She [topic-は] window [object-を] open-throwing-state [adverbial-に] did.
開けっ放し ('left open, open and abandoned') is a productive ~っ放し construction — verb stem + っ放し for 'leaving X done [and unattended].' 出しっ放し ('left out'), つけっ放し ('left on'), 着っ放し ('worn continuously'). The implication is usually slight criticism — leaving things in their done state without finishing or restoring. ~にする here is the 'make into' frame: '[she] made [it] left-open.'