I think what you have to remember here is that a normally healthy and able-bodied person becoming incapacitated (for whatever reason) is a temporary (regardless of the outcome) and UNFORESEEN event. Having a prosthetic limb is a permanent full-time FORESEEN problem carrying risk on a daily basis.
Its a crazy rule because if he only had one eye he would not be allowed a medical so whats the difference with only having one arm? The idea of having two of anything important (engines, eyes, arms, legs) is if you lose one you've still got another - the old belt and braces.
And for those of you using the multi-crew argument to justify this situation please remember this - having a co-pilot is no good if you don't use them. This was a multi-crew operation and it still nearly ended in an accident.
I totally respect this chaps experience and ability and i admire his strength of character and true grit in the face of disability but I don't think he should be granted a medical and shouldn't be flying a commercial aircraft. And before anyone mentions Douglas Bader please remember that he was in an aeroplane ON HIS OWN and there was a war on - a totally different set of circumstances.
On this occasion Flybe were very lucky and they have been very quick to play this event into the long grass but let me ask this - what would be happening now if the aeroplane had have been damaged and people injured/killed? Flybe would be taken to the cleaners.