That guy often has a smile on his face, but he treats customers maliciously.
That lady smiles at customers, but she is kind of underhanded in her interaction with them.
この場合、次のような言い方ができますよ。
ーThat guy often has a smile on his face, but he treats customers maliciously.
「あの人よく笑ってるけど、お客さんに陰湿な対応をする」
to have a smile on one's face で「ニコニコする・顔に笑みを浮かべている」
to treat customers maliciously で「お客さんに陰湿な対応をする」と言えます。
ーThat lady smiles at customers, but she is kind of underhanded in her interaction with them.
「あの人お客さんに笑顔だけど、お客さんに対する対応がちょっと陰湿だ」
to be underhanded in one's interaction with ... で「…とのやりとりが陰険だ・陰湿である」
ご参考まで！
カナダ 
