Does anyone know about a new rule requiring Mexican passport holders to have a tourist visa to Thailand?  I have someone stuck in customs at the airport.

Since end of December 2002 Mexican passport holders must apply for a visa before travelling to Thailand.

Passport holders who can enter Thailand without visa - country list

Not an easy situation your friend has. That person have to either travel back home again, or try to fly to a country that accepts Mexican passports without visa, and try to get a Thai visa there.

How long have your friend been at the airport?

He's only been there for a couple of hours.  Do you think they would consider issuing him one there?

Don't hope too much, but maybe they will. Does he have money or creditcard to show?

It's one of the airlines responsibilities that they check that all passengers have a visa. What airline was it?

American from DFW to Tokyo, United to Bangkok.  Even our express visa service didn't know!

Considering that Thai immigration still have the old list posted on their official web site

Immigration Bureau Thailand

Mexico is listed as 30 days there, hehe. If it was ME I should point towards that website, and try to convince them to let me in anyway.

Ask him to print out that webpage at some internet cafe inside the airpoirt, and show to Thai Immigration. If he speaks polite to them, they could let him in.

Otherwise United have to take him back home free of charge, that's the rules.

Too late.  They gave him a letter of expulsion, and sent him to a windowless hotel room...and now he has to go to Singapore to get his visa and then come back.  So United really has to pay for it all?

Yes.