Hi
Welcome to the forum!
Black toe nails can be due to various causes. One possibility is black toe nail fungus. This can affect those who wear shoes for long periods, have diabetes, are involved in a lot of physical activity and sweat a lot. Local antifungal agents are not enough for treatment. He would need to take a combination of oral anti fungal agents too. Prolonged pressure on the toes as in wrong foot wear or compression of the lumbar nerves supplying the area can cause a discoloration. Poor blood supply as in Raynaud’s disease can be another cause.
Malignant melanoma, benign melanocytic nevi, melanonychia striata (seen in HIV infection, hyperthyroidism, or Cushing's syndrome) are the other rare possibilities causing black color toe nails.
The various possibilities have been listed for you. It is difficult to comment beyond this without examination. Please consult a skin specialist. Hope this helps. Take care!