During that time she had to regard the diary as her only fri

During有两个意思：
1.from the beginning to the end of a period of time 这时不可以用in替代.
2.at some point in a period of time可以用in替代.
My father was killed during the war.
I mentioned the subject during our discussions at her Washington office.
比较：
I met him sometime during the week.（反复,不能用in代替） 我是在这一周的某天遇到他的.
during the holiday 强调假期的从头到尾
He enjoyed himself during his holiday.
in the holiday 强调这段时间其中某一点时间,并不表示自始至终

不可以哦，during表示在这段时间内

no

in和during表一段时间内两词可互用。如：in the night, during the night, in the war, during the war。但略有区别：当接表示“活动”的抽象名词时多用during，接“活动”的动名词及短语时用in。如：
during the discussion
in discussing the problem
during...