What happens to a student visa when the passport expires shortly after?

You can get a new passport any time. There is zero reason to wait until the last year of validity.

okay inside thailand they will only issue an extension of stay that is good until your current passport expires. So what that means for your friend IS

if they're applying for a yearly extension of stay based on education (there is no one year VISA inside the country), they will get it only until their passport expires. AND when they get the new passport they will not get the other 3 months, they will have to apply for a whole new extension and will have lost the time from the other extension because their passport wasn't valid long enough

His visa will not be issued for one year. Suggest he get a new passport ASAP.