。
She'll be back soon.
Literal
[topic-は] before-long will-return probably.
まもなく ('soon, before long') is the more formal/written counterpart to もうすぐ — both express 'imminently,' but まもなく has a slightly literary or announcement-style register. You'll hear it in train station announcements (まもなく電車が参ります 'the train will arrive shortly'), news, and formal contexts. だろう adds speculative confidence — 'will probably.' The sentence is functional and short — a casual report of someone's expected return.