Thanks for all the comments guys.
His right knee is quite weak - he reckons he'd prefer to bend it less if possible; a pedal block/tall shoe is just going to make that worse.
I spoke to him about SPDs (well, one, on the NDS) today, and he was actually very receptive to the idea. He said that when he's tried exercise bikes, his short leg doesn't really do anything other than go round, so he's just looking at getting it comfortable, not using it for power.
So.... Anyone got an old SPD shoe for him to try? left foot, size 10/11? :facepalm: