The original post here related to a person who had "cured" their own schizophrenia using mindfulness. There is quite a lot of research showing that mindfulness can play an effective role in working with schizophrenics. Examples:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4591204/ or
https://ps.psychiatryonline.org/doi/abs/10.1176/appi.ps.002092012?code=ps-siteThe questions regarding negative consequences and participation at retreats are both valid to ask. The first study linked above has a section on negative consequences, "The lack of a significant relationship between mindfulness and negative symptoms may be explained in several ways ..." - i.e. it did not really find any. These studies were undertaken in a supervised clinical setting so that support may account for the lack of consequences found.
Schizophrenia is a set of symptoms and often confused with psychosis - one of the main symptoms. It would probably not be beneficial for a person with psychosis to attend a retreat centre, not for that individual or the other people present. As Stillpointdancer points out, such happenings can be dealt with using compassion and steering people towards getting the help they need.
That brings up the more difficult subject of what Schizophrenia is and what helps. The research that has been undertaken and is being undertaken is showing mindfulness to be of help widely. It reconnects the sufferer with the lived experience of their body and reduces hospitalisations etc.
Unfortunately most of the "help" offered to Schizophrenics and people suffering Psychosis is chemical drugs that squash symptoms. These drugs are poorly understood and fit in with the medicalised model of mental healthcare (which is more about the feelings of inadequacy suffered by the emerging psychiatry profession and pharma companies tinkering with chemicals than real science).
More importantly the drugs used don't just squash symptoms, they also squash the person's ability to relate to themselves and their experience quite radically. Mindfulness works very differently to this: research shows it brings the sufferer into contact with their lived experience and better able to manage symptoms as a result.
Much more research needs to be undertaken. The medicalised model that is current will surely be seen in twenty or thirty years to be quite barbaric I suspect. This suspicion is based on an understanding that Schizophrenia and Psychosis are extreme reactions of the mind to insufferable circumstances. Yes, there are measurable chemical imbalances sometimes, and even changes in brain structure. These too are increasingly being shown to be environmentally induced rather than inherent in the person.
Mindfulness can help people bring symptoms to manageable levels. It can bring them to a place where their mind can face the experiential history and traumas that lead to the collection of symptoms labelled as X, Y or Z by psychiatrists. Facing this and processing it can be achieved with the right environment and grounding in a safer sense of self. And yes, a retreat/centre not aimed at helping such people is probably the incorrect place to attempt this work.