He is taking drugs from the SSRI group, after two weeks suicidal thoughts and deterioration of health began to appear - does this mean that the drugs do not work?

Antidepressants work "with a delay" - i.e. usually some time must pass (usually about 3-4 weeks) before the patient feels full improvement and begins to enter remission. It happens, especially at the beginning of treatment, that patients report a transient deterioration of well-being. In case of severe symptoms (such as suicidal thoughts), I would advise you to consult your doctor as soon as possible. Perhaps, it is necessary to use supportive drugs that will allow you to survive the most difficult stage of treatment. I recommend the article: https://www.gdziepolek.pl/artykuly/depresja