Well, sadly I am unable to find a link to the podcast of a doctor who was on CBC this morning discussing this case.
To paraphrase based on memory (and note my bias per my earlier comments):
Basically, his knee jerk reaction was concern for the health of the woman and the baby.
As he looked into it a bit more he realized that she was already a runner and she effectively walked (or waddled as I prefer to say) the marathon.
He talked, quite interestingly, about hormones that can effect the joints etc... (I think someone mentioned pelvic floor in the posts above which he also discussed in relation to hormones).
Given that the woman was a runner before and during her pregnancy, and given that she was able to continue to run during her pregnancy and complete the marathon, obviously she was not being effected by the hormones in a way that some newbie runner would be.
He pointed out an incident where a pregnant woman started using a stairmaster and ended up being very sore from it (and not just the "first time in awhile" exercise effect).
He also mentioned concern over oxygen and blood flow to the baby (part of his initial gut instinct) - basically, he came down on this being theoretical.
It would seem to make sense but then a plausible argument can be made that with this "redhead" maintaining her running fitness level then she was benefiting her baby throughout the pregnancy with better blood pressure etc, etc.
And, once again, given
her particular history and health, would mean that for her to waddle the marathon isn't the big deal that some people are making it out to be.
He also specified that she is likely one in a million or at least one in a hundred thousand. To do this really requires someone who is already in good shape and who does not have the problems related to the release of the hormones during pregnancy as mentioned above.
I will check again later to see if they post the interview as the whole thing was interesting but what I took away from it is that had he been counselling this "redhead" he would likely have cleared her too upon knowing all of the facts.
Next time this happens I hope it's a brunette so we can get the redhead bias out of the way.