EU VISA Denied boarding QA

[observer_haters]

Hi all

I need your help and guidance. My non-EU spouse booked tickets from Phuket to Brussel through Qatar Airlines. Unfortunately, was stopped and refused to board.She shows them Family Schengen Visa (multi entry), our translated and confirmed Thai marriage certificate and additionally onward ticket from Brussel to Hamburg, She tried to explain the EU rules as well which said:"do not need a return ticket". Unfortunately, rude staff want a return ticket to Thailand! Otherwise, it will not be boarding at all! To avoid the loss already an expensive ticket, I was forced in the last minutes purchase a return ticket from third party company (value 300£) I would like to get your guidance how should I proceed? Any advice would be much appreciated.