As usual, a mix of decent to awful advices. You have two options: 1) Your friend leave before the 90 days mark, pay the fine, and come back the next day via Laos land entry (to avoid problem). Then seek an agent to get him on Non-O. 1) Contact a reputable agent right now. Contrary to what some have said, an agent can help him with such overstay. I've even seen people on much longer overstay not get ban with agent help. Very little is not possible with a reputable agent.
The only legit agent in this group is on the banner above (Thai Visa Centre).
Everyone else and it's a risk. Btw, paying in advance is normal with agent. Sending your passport or wtv to them is also normal if you don't live near. I think I've send my passport 5 or 6 time to TVC in the last 10 years. But "send me X amount and I'll send back in 15 min" is typical Bitcoin scam look-a-like. And having 7 followers is obv a scam. There are legit agent but you need to do your homework.
O Smach is very easy and quick to bounce from. And you don't need to book a hotel, you can bounce in and out in 30 min, so no need to stay overnight. And as others have said, avoid Poipet. I don't care that it's your way to Siem Reap, it's
It's weird to doubt Phuket immigration. It's their job to tell you how and went to get your visa. Nobody here can give you better info then the immigration officer. Brandon is probably right, likely you got the under consideration stamp.
Did you choose Taipei because you want to stay there a week and visit? If not, just take the bus to a neighbor country like Laos or Cambodia, it's much easier to get tourist visa there. Taipei is not a common place for visa runners, but I'm sure some did it this year. Good luck!
You can get it as many times as the embassy in Singapore will allow you. And no problem getting back in. Worse case you call an agent while in airport who will bribe your way in for around 8k baht. This is what we call the VIP service with agent which guaranteed you get in.
OR... after getting the 90 days, you can extend for 1 year with an agent for around 25k baht (without showing 400k). That's the cost for using an agent. You can by-pass the financial requirement with agent. It's up to you.