This is the 535th most frequent Korean word.
도움을
"Help" or "assistance" (accusative form).
Here, '도움을' is the object of the sentence, showing that 'help' is what he asked for.
그는 시험 준비를 위해 친구들에게 도움을 요청했다.
He asked his friends for help to prepare for the exam.
In this sentence, '도움이' refers to 'help', and '필요하다는' is describing the necessity of it.
나는 그녀의 도움이 필요하다는 것을 알았다.
I realized that I needed her help.
'도움이 되는' describes 'helpful', indicating utility or beneficialness.