I'm currently on 90 days single entry Non O and can extend 60 days once. Before, I was on visa exempt and extended once for 30 days and a 2nd time for 60 days. The reason why you can do both on visa exempt or tourist visa is that you first extend for tourism reason but than extend again for a different reason (marriage). But when you enter on Non O you are already on a marriage visa so can only extend for that specific reason.
Do not read your DM, your friend does not need an agent. Your friend can enter Laos today and get 30 days visa exempt. Mukdahan border is hassle free border, they do not ask for anything nor question you. Your friend should have done that right away since he's only gonna be in Thailand for 7-10 days. p.s: As said above, Savannakhet has had a 2 consecutive tourist visa rules for a long time. And also if you have history of volunteer visa you cannot get tourist visa there anymore.
Then why is he doing a tourist visa if he's only going to be in Thailand for 7-10 days? He should have in and out Laos in 30 min hassle free. He can still leave Laos today hassle free and get 30 days.
p.s: Savannakhet only issue 2 consecutives tourist visa, not 3 or more, thats why he got denied. p.s.2: Mukdahan is hassle free border. Wont be ask anything.
Suvarnabhumi is notorious to work hand in hand with agent. There is a 6-8k baht bribe there to get into VIP line for people with suspicious passport (history of ed visa, volunteer, or simply a lot of stamps). So officers there will be tough to indirectly encourage people to use agent. Also, this is Thailand, they do not need a plausible reason to deny you entry. It's up to them if they allow you in or not without a bribe.