I'd be interested if anyone could point me towards a written account of a schizophrenic patient taking psilocybin and being worse for having done so.
This is not the only issue though, is it? My personal opinion is, that, if anything, a psychedelic experience can be helpful in the long term for someone suffering from schizophrenia. In extremely poor taste, you could say that it is not likely to make them go any more crazy

, while the benefits of going through an experience like that and its potential for healing are probably obvious to everyone in here.
At the same time, if we accept that psychedelics are mind-manifesting, and we also accept that the mind of a schizophreniac contains stuff that is obviously very difficult to work through (given that they were troubled by it enough to seek psychiatric help), the potential for short-term trouble during the experience is obvious.
I was once accompanying a friend diagnosed with schizophrenia while she tripped on LSD. It was very intense for her, but overall, she loved it, and even after several months have passed, was happy to have done it. At the same time, she also ran off away from the group during her trip and was found walking down the middle of the road, being quite loud as well. To clarify, she did not have suicidal intent, and was in good humour, she just did not think of the complications that might arise from such conduct as she was simply too busy taking it all in at the moment, so to speak.
All's well that ends well, it was nighttime and the parameters of said road were such that she was unlikely to be hit by a passing car, but if she ran into a police patrol before being found and returned to the fold, she would have spent the rest of the trip locked up and probably injected with antipsychotics for good measure, followed by a several weeks long stay in a mental hospital (not that it was neccessary, but she had a history with mental hospitals and would have been kept for observation. While being sedated with AP medication throughout, to stymie her rightful indignation at being deprived of her freedom. This passes for modern psychiatry in some places. But I digress.)
This could probably happen to anybody given the right (wrong) circumstances, but I stand by my claim that schizophreniacs are more likely to get into trouble on psychedelics. This has been my experience and it simply makes sense, IMO. This is no reason to treat people with schizophrenia like subhumans and deny them something you would otherwise happily share with them without any second thoughts if it weren't for their diagnosis, but it's prudent to be aware of the danger and take any precautions that might help avoid a train wreck scenario.