What should my friend with an ED visa know about re-entry into Thailand from Vietnam?

If he’s been going to class get a re entry permit and he should be fine but if the IO sees something in his history you just don’t know totally up to them

Is it for university study, or language school? University is fine, but any informal education is recommended to NEVER try leaving and re-entering.

What John said.

To be honest, I don't like the recommendations with the agents for "safe" entry. The majority is complaining about the fact that almost everything in Thailand needs tea money but then you read every time "contact an agent who's helping you with the entry". Those agents also "help" with paying someone at the immigration, this way the method will never really end.

Your friend will very likely not be allowed back in even with a re-entry permit. Please contact an agent for safe passage. If agents says that it's not possible in Bangkok, but in Phuket, Chiang Mai, etc, make sure to avoid Bangkok airport.

Your safest bet is to contact Thai Visa Centre - visa agent. If they tell you that it's fine with their service then it is. It they tell you that they can't even take your friend as client, then tell your friend to not travel with you. if Thai Visa Centre can't do, no other agent can. TVC have a lot of contacts with immigration at most airports.

No idea about you as you don't say what sort of visa you are on, but your friend with the ED visa will need a re-entry permit and to keep his fingers crossed that he doesn't get denied entry on returning to Thailand. It's not noise, immigration don't take kindly to people leaving the country while they are meant to be studying, and even those with letters from their school get denied. Will your friend be denied? It will be entirely up to the immigration officer they will stand in front of