。
She couldn't wait for him to come back.
Literal
She [topic-は] [he [subject-が] [come-back-帰ってくる] [nominalizer-のが]] couldn't-wait.
待ちきれない is a compound verb: 待つ ('wait') + ~きる (do completely) + ない (negative) — 'can't wait through it' = 'can't stand the wait.' The ~きる is often translated as 'completely,' but the negative form means 'unable to bear/hold out for.' The embedded event 彼が帰ってくる is nominalized with の and marked が as the subject of the can't-wait predicate.