Being granted a visa does not give right of entry. Immigration decides at port of entry. Embassies are not linked to immigration.
As I said before, I asked this question here a while back for a friend. I consulted 2 lawyers here, a Thai one and a German one. Both confirmed there was no way to reliably find out prior to arrival. The land crossing seemed the simplest test. Fly to, eg Phnom Penh, and choose a quiet border checkpoint. If he gets in, do what he needs to do ASAP and leave. He could still be busted for illegal entry.
My 2¢ worth. YMMV.