That person could speak to a number of people who testified before a parliamentary committee in Quebec before Quebec decided not to allow medical assistance in dying for people suffering solely from a mental illness. The committee heard from individuals who were in exactly the situation you're describing. They told the committee that, if medical assistance in dying had been available to people with mental health issues, they would be dead, because they met all the requirements three years earlier. They said that they were now very happy to be alive and that they had received good treatment.
If it were possible to predict who will be happy to be alive in a year or two and who will continue to suffer, that would be different. However, so far, all the research indicates that this isn't the case. Even research on schizophrenia indicates that symptoms lessen with age.