My friend needs to re enter china with 90day chinese visa, is this possible at seoul embassy?

First, visa can't be extended once expired. So your friend has to apply for a new China visa in fact. If you friend plan to apply for the visa from the China Embassy or Consulate in in South Korea, he/she must provide the alien registration card in South Korea with at least 6 months remaining validity. It takes about one week to apply for a visa. The visa fee is different for applicants from different countries.

By the way, the China Embassy or Consulate in in South Korea doesn't accept individual applications of ordinary passport holders, and your friend has to apply for the visa through travel agencies designated by the Consular Section of the Chinese Embassy.