。
She thought his offer over carefully.
Literal
She [topic-は] his [genitive-の] offer [object-を] carefully thought.
よく has many faces — 'often, well, carefully.' Here, modifying 考える ('think'), it means 'thoroughly, carefully, well.' Context disambiguates: よく考える ('think well/carefully') is one of the standard よく+verb collocations, meaning to weigh, consider deeply. Compare with the frequency reading よく行く ('go often').