。
She decided not to go.
Literal
She [topic-は] not-go [as-it-こと-に] decided.
~ことにする ('decide on, choose to') is the personal-decision counterpart of ~ことになる ('it is decided that, it has come to pass that'). The pattern attaches to a plain-form verb: 行くことにする ('decide to go'), 行かないことにする ('decide not to go'). The の/こと nominalization wraps the verbal clause as a noun, then に+する treats it as 'set [it] to be the case.' Past 決めた makes it a completed decision.