The psychiatrist or therapist would diagnose accordingly.
There are many passing or longer-term mental / emotional health conditions relating to a person's experience of difficult circumstances that are not psychotic in nature, but nonetheless fall within a medical remit, such as depression or anxiety, and adjustment disorders (disorders of mental or emotional state in an otherwise mentally healthy person, currently adjusting to major trauma or stress).
It would have been perfectly reasonable and honest of a medical health professional to attest in writing to Kate's suffering from one or more of the above, judging by the various accounts we have of her reaction to her situation. If it is true that Gerry had to take time off work to look after her, for example, as one of the witnesses reported, this would be evidence that she was temporarily unable to function. That is reasonably serious, without coming under any classification of psychosis.
Although I must admit this is all more Dr Watson's line....
If a person who was suffering from depression went to their GP the GP would make a judgement on the severity of symtoms described
He/She might think it appropriate to use drug therapies combined with counselling, or councelling alone, depending on the professional judgement made
If a person went to their GP suffering from depression that was of such severity they were having suicidal thoughts, the GP would, without question, arrange an immediate assessment at a psychiatric unit
That would be essential in order to assess whether the patient presented an actual 'suicide risk'
I do not understand why, if Kate was having suicidal thoughts because of Amaral's book, this normal procedure was not followed ... and the medical records pertaining to it produced in court as evidence