Has your DC considered multitracking into GarageBand or similar? He could divide parts written for two hands into more hands (or the equivalent for his instrument), or play it slower and speed it up. Even more fun, try writing orchestrations that articulate the challenging parts into multiple instruments. One of my DCs has a similar issue with composing music that requires a great deal of practice to perform, and has found that writing orchestrations allows additional musical elements to emerge from one's head.
Also, he would not be the first composer to write music that was technically too difficult for himself to play. Robert Schumann, as you likely know, damaged his hands trying to "stretch" his span, which is why he was extremely fortunate that his wife Clara was an even more talented pianist than he was, and could perform his works. It's a not unheard of opinion among critics that Rachmaninoff (as skilled a pianist as he was) was not the best interpreter of his own works.
