Does it violate some code to comment on a game you're observing?  Its allowed evidently.  Suppose its a match of a chess.com friend and you say something like, "Looks like he's playing a modern defense." 

I believe your comments should not be of the type that give away a strategy the opponent may not have figured out.  (Ex. - "I see you're looking to mate on the Queen's side...")  Perhaps a note of encouragement to whichever player you think made a good move might be OK.

Generally, though, I would stay away from making comments at all.