Actaully, while the racing rules state that a racer cannot accept help or equipment, if a racer does accept either, it is up to the racer to report it and DQ him/herself, or up to another racer to report it.

Once saw Nick Waite DQ himself after having 3 flats, accepting 1 tube because he only had 2 with him, and STILL cross the finish line 5 minutes ahead of the next expert/pro racer........

If people are just riding, I would find it the height of rudeness for either of my kids NOT to stop and help somebody; during a race, I would - at the very LEAST - expect them to ask if the rider was ok, and to report the problem to the next course marshal they encounter. But, since my kids have grown up in the WV MTB community, I don't have to worry about them being idiots out there - they always help or offer assistance, and would rather run their bike than accept help.