You friend arrived without any visa and used the 30 day Visa Exempt Scheme.
If he did not contact Immigration during his stay, he will be in overstay and face a 500 thb a day fine with maximum 20.000 Thb. Depending on reporting himself at airport Immigration or beeing caught there are other issues to solve.
Fisrt off all, your University is only providing documents to apply for a visa or extension of stay at the Immigration Office.
You get OR a Non Immigrant ED visa which gives you 90 days of stay OR an
Extension of Stay depending on course length with maximum 1 year, what ever comes first.
If you like to leave during this issued stay, you buy a re-entry permit at the Immigration Office or last moment on some International airports. This re-entry permit is valid until and will give you the same date as mentioned in your passport as permitted until xx/xx/xx
I did not read any exception for AirBnB house owners:
The law: According to section 38 of the 1979 immigration act, "Land owners House owners, the possessor, heads of household, landlords or managers of hotels who accommodate foreign nationals on a temporary basis who stay in the kingdom legally, must notify the local immigration authorities within 24 hours from the time of arrival of the foreign national." If there is no immigration office in the province or locality of the respective house or hotel, the notification is made to the local police station.