What documents are needed to extend a tourist visa in Thailand and when should my friend apply for the extension?

To extend a tourist visa in Thailand, your friend should visit the immigration office when there are 30 days or less remaining on the visa. In major cities like Bangkok, Phuket, and Chiang Mai, they can also apply when they have up to 45 days left. It's advisable to go early since the extension adds additional days without any negative impact.