Can my friend change his education visa from a language school to a master's program while in Thailand?

He wouldn't be changing any visa because he already has ED.

The biggest thing is if he's on the original 90 day ED visa, or if he's on an extension of stay based on education. If the last stamp in his passport says "extension of stay" then he's on an extension. If that's the case he "might" be able to change to the university without leaving Thailand. He will need to ask his school and ask his immigration office if they will allow him to change his extension from the language school to the university. He would need to get a letter from his language school advising the last day of study and then cancel his existing extension and then at the same time apply for a new extension based on the new school. He will need all the paperwork required from the school in order to do this. If it's 2 different immigration offices then he'll have to coordinate between the schools, and both immigration offices and ask if they will all cooperate.

If he's still on the initial 90 day ED visa then there's nothing he can do. He will need to leave Thailand and apply for a new visa.