A friend in Vietnam was planning to visit Bangkok and visited the Air Asia office in HCM to buy a ticket. While there he was told he would need to prove he had $400 (or equivalent) before he would be allowed to board the plane. This policy (if true) must have been mandated by Thailand. Has anyone else heard of this ?

It has always been a requirement for entry into Thailand. It may not often be checked but it can be and sometimes is so if airline has any doubts they can require such a show.

Visitors over 12 years of age are required to hold

sufficient funds to cover their stay (at least THB 10,000.-

per person or THB 20,000.- per family).

http://www.delta.com/planning_reservations/plan_flight/international_travel_information/visa_passport_information/index.jsp

Foreigners entering Thailand under the Tourist Visa Exemption category must possess adequate finances for the duration of stay in Thailand (i.e., cash 10,000 Baht per person and 20,000 Baht per family).

http://www.mfa.go.th/web/2482.php?id=2490