。
She's used to getting up early.
Literal
She [topic-は] early-rising [to-に] has-become-accustomed.
~に慣れる means 'to get used to something,' with に marking the thing being adapted to. The ~ている form here indicates a resulting state — she has completed the process of getting used to it and remains accustomed. The noun 早起き plugs directly into the ~に慣れる frame as the object of habituation.