Q:

にする in this sentence?

A character asked her what she was doing "here", and she answered :

捜査中に誰かを1人にするのは、極力避けようって話だったでしょ？

I don't understand the part with にする, what does that mean ?

A:

「[1人]{ひとり}にする」 here means "to leave someone alone".  
It should be remembered as a set phrase, but if you need to understand it grammatically, 「～～する」 here means "to make/let/leave one ~~".

「捜査中{そうさちゅう}に誰{だれ}かを1人にするのは、極力{きょくりょく}[避]{さ}けようって話{はなし}だったでしょ？」

thus means:

"We all agreed to do our best not to leave a person alone during the investigation, didn't we?"

