@Karel ****
if he enters on an STV, and tries to convert it to another visa type, it *may* result in him getting thrown out and blacklisted from Thailand.
If your friend enters on a visa exempt, most immigration offices will not allow a person to purchase a single entry non-o visa based on marriage inside Thailand.
If your friend enters Thailand on a 60 day tourist visa, he *may* be able to get a one year extension of stay based on marriage to a Thai national.