。
She has been living alone for a very long time.
Literal
She [topic-は] very long interval one-person [as-で] is-living.
暮らしている marks the durative state of living somewhere — a long-term, ongoing condition rather than a momentary action. 一人で uses で as a manner/condition marker ('in the state of being one person'), giving 'alone, by oneself.' 長い間 ('for a long interval') is the standard duration phrase paired with these state-verbs.