Yeah, wearing mask all time is one of the best way to prevent infection and I'm thinking of how to avoid that dinner. He just stays in Phnom Penh for one evening and I think I can toss him a hamburger and a pizza. The reason is he just come back from the west and might not get used to with our foods and he will need another trip to the Vietnamese border tomorrow, so there's no room for error.
I'm not sure if there is any PCR test requirement before boarding. Giving the altitude of the west toward the pandemic, I guess "NO". However, there is no PCR test or quarantine required upon arrival of one is fully vaccinated.
Sound like you have a plan. It is only one night and a little sacrifice. Toss in some bun and snacks if needed.
Impersonal perhaps but better be safe than sorry.