Can you really get details about the commenter using the comment_text action in wordpress?

What I wish to do is modify a comment someone makes according to who they really are, like if they're user_a I may want to make their comments appear as eco-friendly, if they're user_b I would want their comments to become bolded, or formatted in a different way.

Make use of the function get_comment_text

$comment = get_comment_text();

Untested, truly a function prefixed by "get_" in wordpress returns the worthiness rather than directly echoing it.

Anyway, you'll need the author's information too, so you've to obtain the comments via get_comments and loop through them.

$comments = get_comments('post_id=15');
  foreach($comments as $comm) :