Read the contract of carriage carefully. You are legally responsible for all visa requirements and if refused entry pay the return airfare. . However for the moment Thai immigration said you must leave and it is the airlines reposnsibility to fly you out even if they have to bump another passenger. They can't literally twist your arm and make you pay on the spot but they can take action to recover their losses later.
Truly. You could be let in then “change your mind” get an immigration lawyer to plead for change of status and stay forever. It would be looked at as gaming the system and I guess a black mark against the consular officer. I have a perfectly respectable friend who tried to get a tourist visa three times. Her late husband wanted to take her to Hawaii. He was KOREAN so there is no way she could pull a fast one. When they finally got to the US she was separated out in an immigration inspection room and treated rudely. They told her not to run away like the first thing any Thai woman wants to do is leave her job and husband and go work in a Waikiki massage parlor. An absolute disgrace.
Is it possible for her to to apply on her own merits and not mention a boyfriend at all? Just say she wants to visit Disneyland? I have been told (informally) that a Thai woman with approximately 2.5 million in bank or her parents if she is a student with strong reason to return can likely get a tourist visa.
With the “ foreign insurance verification form” it is possible if you meet the requirements. For me with AETNA, after 3 months of emails it was not possible. I had to buy redundant insurance from a Thai company.
Friend had a sudden credit card problem flying from Phnom Penh on PG. I had to book him a outbound flight on my mobile or he would have missed his flight!