He wrote the story with his friend's 'assistance'. = He wrote the story 'with the assistance of' his friend. [=his friend helped him to write the story]
Our family had to 'go on public assistance' [=receive money from the government] when my parents lost their jobs.
Nobody 'came to her assistance' [=nobody helped her] when her car broke down.
(..) but she knows how to present herself. A pretty assistant is the most effective form of misdirection. Thank you. Ladies and gentlemen, for my final trick, I will require the assistance of two volunteers. Mr. Merrit, if you would oblige me? Yep. - Under? - Under, yeah. (..)2006 The Prestige