Have I understood correctly that a 1-month tourist visa can be extended once for another month?

Thanks

Probably not, but depends on your nationality. A 1 month tourist visa you get at the airport is a visa exempt entry and cannot be extended, but they might give you an extra 7 days if you can show a ticket out within those 7 days.

A real tourist visa, which is for 60 days, can be extended.

But you don't state your nationality. If you belong to a nationality that has a tourist visa for 30 days from a Thai consulate (and not from the airport) then you can extend it.

Actually the extension is not for me, but for a Cambodian who got the tourist visa from the Embassy in Phnom Penh

Than he should be able to extend.