Guess I should've read the rulebook. It's in there, sort of.
3D3. The following are alternative methods for handling lapped riders in criteriums (i.e. riders who have been caught by the lead rider(s) in the race). The method chosen by the organizer with the Chief Referee must be clearly explained to the riders prior to the start of the race.
(a) A rider who falls so far behind as to be considered out of contention may be removed from the race by the Chief Referee.
(b) Alternatively, lapped riders may be permitted to remain in the race and all will finish on the same lap as the leaders. At the finish, these riders will be placed according to the number of the laps they are down and then their position in the finish.
3D4. Riders who have lost contact with the field, and are then caught by a breakaway from the field, may not lead [disqualification]. Riders off the front of the field may not accept assistance from riders who have lost contact with the back of the field [disqualification for accepting such assistance]. Lapped riders may rejoin and race with the field.
Obviously we were working in situation (b) this weekend, with riders remaining in. So you're allowed to join and work with the "field", but not give assistance or lead a group "off the front of the field".
So basically, our rider should've been DQ'd for working in the break, and all the riders in the break who drafted him should've been DQ'd.
This race was split into about 3 main groups. Each was about the same size as the other. So what's the "field" in this situation?