The chance to allow a special residence permit because of health problems is not connected to the chance of obtaining cures in Italy but it is connected to the trravel back home and possible occuring health problems. Consolidated act art. 35 entitles clandestines to health services and this is valid during the whole of the stay in Italy.
Rules allowing essential and fundamental healh services is not obstative to expulsion. Travel can make the difference, this means that in case the person cannot possibly move health matters residence papers can be released.
The chance of having a health matters residence papers released because of psychic problems is quite remote in fact nothing would stop an expulsion.
I am not a doctor and I therefore cannot judge the situation, I believe that staying inside a detention centre won’t help the Nigerian man. It will be the doctors to state whether moving from Italy would be dangerous or not.