I am occasionally asked by other students in my research group to read their manuscript/poster before it is submitted/presented. I would assume that the main reason to get another person in the same field to read your work before submitting it is to check that the content comes across clearly and without errors (although in my case I am also one of the native English speakers in the group so this might also be a factor) and that this is something different from the responsibility that the editor has.
To what extent is it acceptable to:
(1) make suggestions for fixing frequent, small grammar issues (missing articles, noun-verb disagreement etc.)
(2) make suggestions for modifying formatting even where it does not affect understanding of the work (e.g. inconsistent capitalization of labels in figures, bulleted lists that are not consistently either full sentences or clauses)?
I guess that the consensus is (1) is acceptable and (2) is less so, but I would be interested to hear other opinions as I am always somewhat uncomfortable making a large number of trivial suggestions when the content of the work is good and I find limited real issues to comment on.