It's the responsibility of the airline as they can be fined for allowing passengers on board without an outbound flight or the right types of visa that may exempt them. Some airlines are more pedantic than others so it pays to check. Possibly risk it and it's easy to buy an outbound flight at the counter if they call you out.
James Reid Patrick if push came to shove that could be very problematic for him not to say expensive. Might not do much for your relationship but then with friends like that why not...m
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.