Hmm. I'm not an expert, but I suspect it all come down to the nature of the prosthetic leg. Would he be wanting to cycle wearing it, how well will it bend at the knee etc.
IF (and I don't know much about false legs), it can act like a normal leg, bending at the knee and ankle, and it was comfortable enough at the join to his real leg, then he could possibly just have the foot strapped to the pedal. If not, or if the interface was too bulky or uncomfortable, he might be better not wearing it and pedalling one footed, clipped in. With a trike at least, there is no issue of having to get a foot out of the pedal and down when stopping. He could carry the limb on the trike, and attach it when he got there. It depends how much upper leg he has left to need supporting.
I do know of two people with artificial legs who cycle - both uprights and both I think just clipped in as if it were a normal leg. One of them had to have a very fancy electronic prosthesis (amputee at the hip) to allow for the knee bending aspect, but I don't know the details. The other, I think, was a lower leg only. I have also seen a man in York on an upright who has his cranks adapted so that only one turns (with foot clipped in), while his other leg rests on the other pedal at the bottom of the stroke - I don't even know if that's a false leg, or that his own leg is immobile.
The main thing is, I don't think the trike would
necessarily need a a lot of alterations - but it's probably something an expert would assess on a case to case basis.
Whereabouts is your friend? There is probably a local organisation who specialise in adapting bike and trikes, who could provide more detailed advice. Have a look at this page for some hopefully useful contacts.
http://www.velovision.com/cgi-bin/show_comments.pl?storynum=559
