If the story were true, and the alleged drug trafficker had been hoodwinked by criminal masterminds, then this is a gross injustice done against him. Unfortunately we cannot do anything about it as we cannot interfere in the legal system of another country. When I say "cannot" I mean it in the literal sense - we are unable to.
Then again, the Chinese authorities have a predisposition for execution, as they have immense gains and profits to make out of it (which I find sickening) such as organs. It is probable that they did not want to make an investigation into the man's supposed insanity because they have a financial motive to kill the man.
This is due to the loophole in the Chinese legal code, that organs cannot be harvested from a person without consent - UNLESS that person is a prisoner.
This is a really sad situation, and we cannot do anything about it - any intervention could spark tension between two militarily powerful parties (China and Britain/US), which is really dangerous for the world.