NSF Graduate Research Fellowship, how much input from one's own advisor is ethical?
There is nothing wrong with having assistance on this or any other fellowship application.  It would be foolish not to seek out assistance.  Note that the NSF even advises "get feedback from others" and provides a list of experienced people who can help applicants.  The expectation is that applicants will have help.

While there may be a level of assistance that is inappropriate, someone who "worked extremely hard on his application" should not spend a moment worrying about this.  The complaints from his peers sound like jealousy.