。
She did the job by herself.
Literal
She [topic-は] that work [object-を] one-person [in-で] did.
一人で ('alone, by oneself') uses で in its manner/state sense — 'in the state of being alone.' やる ('to do') is the casual counterpart of する; in this context it describes hands-on completion of a task, often with a slight sense of self-reliance or pride. The whole sentence captures self-sufficiency, resilience, or perhaps complaint, depending on tone — 'she handled it on her own.'