Maximum 90 days stay

Dear All,

I am in Korea now and my friend from the Czech Republic is going to Korea for 90 day. He could be here for 90 days maximum. But after 90 days in Korea he is going to Taiwan or Japan. When he comes back after 1 week from Taiwan or Japan, could he be in Korea next 90 days?

Thank you very much for your help.

Kind regards,

Martina