。
She's used to living alone.
Literal
She [topic-は] solo-living [to-に] is-accustomed.
一人暮らし is the standard noun for 'living alone, solo living' — a concept of growing cultural weight in Japan, where multi-generational households were once the default and one-person households now make up the largest share of all dwellings. 慣れる ('to get used to') takes に for the thing one becomes accustomed to. ~ている here is resultative: she got used to it (point-of-change) and the resulting state continues — a textbook use of ~ている with a change-of-state verb.