。
She was distant the entire time.
Literal
She [topic-は] the-whole-time stranger-like was.
他人行儀 ('formal / aloof / overly polite for a familiar relationship') is a four-character compound: 他人 ('other person, stranger') + 行儀 ('manners, behavior'). The phrase describes someone treating an intimate or familiar person with the polite distance typically reserved for strangers — a behavior often gently called out in Japanese. ずっと ('the whole time / throughout / by far') signals duration or intensity; here it's the durative reading. The behavior is contrary to the Japanese ideal of 親しき仲にも礼儀あり ('even between intimates there's etiquette') only because the etiquette in question has tipped over into chilly formality.