LondonYouth - thank you for your post. I don't think anyone here is calling your organisation into disrepute. It's a shame you had an issue with your athlete this year. Perhaps she might rectify your name being dragged through the mud by matching the donations she's received on her JG page by way of apology to you? One can hope.
I hope that her personally negative publicity draws some people's attention your way so that you gain something more positive out of this in terms of wider awareness of your cause. Best of luck to you.
I do have some sympathy for the athlete, we don't know their circumstances. If there had been a fatal accident closing the M1 for several hours, for example, then that's very difficult to overcome. On such rare occasions some events will keep registration open a bit longer for the affected athletes. If it's a one off set of circumstances particular to them, the athlete could try contacting the event before the expo closes (as they are likely to know that this issue has arisen) but unless it's life or death (in which case the athlete probably would prefer to defer anyway) it's quite difficult for a massive event like this to help. Their staff will have been at that expo for several days and are likely to have other jobs to move onto once the expo closes. Also, what's justifiable to one person, isn't to another. Like Cougie says, you have to draw a line somewhere and this is an event with 35-40 thousand participants who have had several days in order to register.
Sadly, I think it's a bit like catching a very important flight. You allow two hours to check in, then you add on an extra hour for contingencies and if you want to be super sure you add an extra hour on top of that. You can't leave anything to chance. I quote back to you, the athlete has made all that effort to train and raise funds, the athlete needs to be super organised to ensure they can make it to the expo, allowing time for issues.