oh poor dd. I am not sure about re-doing the test, but I would have thought they would have written soemthing down about where she was going wrong. Then if I were you, i wouldnt make a big deal out of it but maybe take her for a few bikes rides and go over the bit she found tricky. She will soon pick it up.
I have never come across a situation where they have needed to provide anyone with their certificate. It just goes in the 'box' with all the other bits and peices we have no place for
What a shame. It's only fair that she should be told what she did wrong, so she can work on it, whether she can retake or not. Has she remembered anything since Friday? Maybe her nerves just made her wobbly, in which case she's probably a perfectly safe cyclist in normal life. I think it would be reasonable to ask the school if they can find out from the examiner why she failed - for safety reasons.
Has she actually been told she has failed? Last year a couple of children at our school were told that there were a few things they still had to work on,they were given a sheet the same as all the others,but it showed them where work still had to be done. A few weeks later they were seen again and everything was fine,seems very unkind to fail her on what probably was nerves.