I saw a video of the finish of the women's 10K at the Flying Pig Marathon. The leader is walking about 20 yards from the finish. The woman in second stops and starts to pull the leader along, and the leader falls. A race official runs out and helps pick the leader up, as does the woman in second.
http://www.youtube.com/watch?v=ThlAenn6yV0&feature=player_embedded
I believe that the official providing help would cause a disqualification. What about the aid from someone competing in the same race, i.e., the woman in second? I think the woman in the lead would have finished, albeit in second, without falling had she just be left alone.