。
She was completely fooled by his smile.
Literal
She [topic-は] his [genitive-の] smile [by-に] completely was-deceived.
完全に ('completely, perfectly') is the adverbial form of 完全 ('complete, perfect'), modifying 騙される. The に on 笑顔 ('smile') marks the agent of the passive — what did the deceiving. 騙される (passive of 騙す, 'to deceive, trick, cheat') here gets a sympathetic reading — the smile lulled her into being fooled.