Ha, CX/KA can't win or lose with this one! I distinctly recall we've had a thread where a poster was furious at CX/KA for delaying the check-in process on account of zealously checking arrival documentation.

As Seawolf mentions, the OP is absolutely not wise to write a letter. CX could potentially be fined by the Thai authorities, and CX is within its right to force the passenger to reimburse! Although it's a crap situation, it's completely the passenger's fault. That's just the way it works.