What steps should my friend take to switch from an ED visa to a DTV visa in Thailand?

Another candidate for the cooking school. At this rate within five years every foreigner will be a Master Chef! 😂

Make sure your friends gets a letter from the school stating when they'll finish their studies and take that to the immigration office that issued their extension to get it cancelled. They can go about 2 weeks before the date and the immigration office will cancel their extension and stamp then in until the date on the letter.

Then they'll be clear to apply for the DTV outside of Thailand and won't have to worry about an uncanceled education extension causing them issues and fines in the future.