Student visa duration longer than school duration

Hi all!
My friend has started a 6 months language school in Japan. However his CoE stated 15 months (a normal practice says the school), and his student visa also run for 15 months.

Has anyone been in a similar situation? Can my friend stay in the country once the 6 months of language school are over? Or should he reach out to immigration to change his visa status after 6 months?

Thanks!