How acceptable is this phrase in Norwegian? Is it the accepted/standard/inoffensive way to refer to mental illness?
To keep it quite neutral and standard, I would use "De er psykisk syke", or "De har en psykisk sykdom" instead. The original sentence is not rude, but it might stress the illness part a bit too much. Also, I'm not sure why it says "på", to me it would be more grammatical to have "De er syke i sinnet."