Left fielding again - but do you have any obligation, or right to report?
Are you not just another 'spectator' at said match, in the overall scheme of things? You are not an appointed official for this game, and as we've already said elsewhere the "I am a referee too" argument when dealing with players etc in games so would the referee in question be within his/her rights to dismiss your comments in the same vein? Perhaps.
Indeed, can you prove that they weren't sent in? Have you ever been asked not to send in and said nothing at all? Sometimes this is interpreted as "I've had a word and he won't put it through." when in fact the opposite is true.
I am not condoning the actions of the other referee, but nobody likes a tattle-tale, or dare I say snitch? Not necessarily my opinion, but there will be people who view you as this.You can damage yourself by getting a reputation as the stab-in-the-back guy which undermines you as a referee amongst your colleagues.
If you're involved with a club however then the game changes. "Dear disciplinary department. My player Piece Oftheaction was booked in the game against FC Nomarks. I've not seen anything from you yet. Can you please advise?" The referee gets the message from the CFA, starting with a did you forget to send it in? before building up to a formal reminder or responsibilities.
They get found out; they get caught and lose their games eventually. It doesn't make our lives easier if they don't send them in but that is how it is.