。
She frowned the moment she saw the dog.
Literal
She [topic-は] dog [object-を] see [whenever-と] face [object-を] frowned.
見ると uses the conditional/temporal と — the 'reliable trigger' connector that means 'whenever / as soon as / and then.' In narrative writing this often slides into a 'discovery' reading: 'she looked, and (as a result) she frowned.' The idiom 顔を顰める ('to wrinkle one's face,' 'to frown, scowl, grimace') is the standard way to describe a displeased expression; 顰める is one of those verbs you'll mostly meet in this collocation. The kanji 顰 is rare enough that it's often written 顔をしかめる in modern prose.