It was in the factory----his father worked for 10 years ----i met Mary for the first time.

where,that
前一个兼作定从的地状，后一个是强调句型的固定搭配

where,
that(因为是强调句,后半句是强调重点)

我和玛利的首次懈逅发生在她父亲工作了十年的厂里。(翻译时句子顺序调整一下以适当汉语的习惯)

where,that.第一个是状语从句，针对in the factory而言，表示地点，所以是where，第二个是It is …that的句示，是强调句，固定用法。

1.It was in the factory（ ）his father worked for 10 years
第一个where 地点状语从句

It was in the factory （ ）i met Mary for the first time
第二个that，固定搭配 it was the first time that(第一次做某事）

where 地点状语从句
that 强调句