。
She gulped down the beer in one swig.
Literal
She [topic-は] that beer [object-を] in-a-gulp drank-up.
飲みほす is a compound verb pairing 飲む ('drink') with 干す ('drain dry') — to drink to the bottom, finish off completely. Compound verbs of this shape (食べきる 'eat up', 使い果たす 'use up') express thorough completion. グイと is a 擬態語 mimetic for a forceful single-action gulp or pull — same family as ぐいっと from rope-tugging, applied here to liquid. Beer specifically attracts drinking-mimetics in Japanese: グビグビ (chugging), グイッと (a satisfying single swig), クーッと (a refreshing first sip). These auditory color words make Japanese drinking culture sound like a sport.