Not going to comment on this particular case (I tend to sign these forms, where needed, though might have to charge as outline below) but there are some additional things to consider-
Just think if all events of this nature needed a medical confirmation, and then think how much GP time would be taken up if every person registered with a GP needed a form signing for every event they did. Not only this, but should a GP's time be taken up doing this work? This is not classed as NHS work and takes up appointments for people with genuine illness. Since it is non NHS work, and relies on several years of training and expertise, and expensive indemnity cover, it may be entirely justifiable to charge for this work. A solicitor would charge for his signature no?
It's a nonsensical situation not to be able to certify our own self fit for activity, or at least declare that one has not been declared 'unfit'. As participants in this sort of activity we should try and resist the increasing situation where we need to get a 'medical opinion' before participating- this is nonsense unless the participant themselves has genuine personal concerns and needs to seek advice on this basis.
It is impossible to give a medical opinion that guarantees someone will survive a physcially demanding event- spontaneous events do happen and people drop dead- and a doctor may not be able to, in a single appointment necessarily perform the examination, tests and formulate an opinion that someone is medically fit. In fact, without an ECG and possibly further tests, clinical examination alone may not be sufficient for any GP to offer a valid opinion of fitness- and hence there is increasingly advice not to sign such forms that require such an opinion based on examination alone.
However, that said, I'm entirely encouraging of sporting activity and do not wish to present an obstacle to people participating in such events, and try to be as helpful as possible.
I'm purposely not trying to comment on the above- but just to say there are some additional considerations