。
She'll probably be happy when she gets engaged.
Literal
She [topic-は] get-engaged [if/when-たら] happy will-become probably.
幸せ ('happy, happiness') is the warmer, more everyday Japanese word for happiness — drawn from native morphology (the verb 幸す, archaic 'to bless'), softer in tone, and far more common in spoken contexts and personal expressions than its Sino-Japanese alter ego 幸福. 幸福 sounds more like a philosophical or institutional concept (think 'felicity' or 'wellbeing'); 幸せ feels lived-in, like 'happy' in everyday English. The conditional ~たら attaches to past-tense verb forms (婚約したら) to mean 'if/when X happens,' and the polite conjectural でしょう closes with a softened prediction. The whole sentence reads like a tender, well-wishing remark.