The London website says several thing, but there’s no official statement regarding this. There’s no official limit on entering by air on visa exemption, but sooner or later you’ll be pulled aside and told to get a proper visa or denied.
Visa into Laos is by road is approximately $35. And he doesn’t need to sleep there. This is how border runs are done. If he doesn’t have any previous history travelling in and out of Thailand this should be fine.
As said, if you just want the new 30 days exemption, it’s very convenient to use a Land border transport company. There are several in the tourist spots and in Bangkok. They are very experienced and it’s done just in some hours every day at 3- 3500 thb including everything.
Ok, with a French passport he’s entitled to a 30 days visa exemption at any border into Thailand. The general rule are two entries by land in a calendar year and some more by airports. Is no specific amount of time you’ll need to be out of country to get another or a new visa exemption. Normally you just stamp out of Thailand, into a nearby country and return. If he don’t have any previous history travelling in and out of Thailand, this should be no issues. If you just want the new 30 days exemption and don’t have any plans for a trip outside Thailand anyway, I’ll recommend to just use a land border transport company where you stay. It’s normally just done in some hours within one day and it’s not expensive.
You’re from Bulgaria and you’re only entitled to 15 days visa on arrival, which means you’ll need to arrive by an international airport. How long you’ll need to be away for to apply for another VOA, it’s not easy to say. But if you plan to take a trip to Laos, it could be a better option for you to book an appointment online before you leave and apply for a 60 days tourist visa at the Thai Embassy in Vientiane. Normally done in two-three days.
If you’re allowed to apply for a 90 days Non O visa directly on your current visa, it’s TM.86. After 90 days you’ll do an extension of stay on a TM.7. You for sure can’t go directly from a ED-visa to an extension of stay.