Bondi
If as you put it Norwegian "returned" a crew member to OSM then the crew members dispute would be for OSM to resolve as they are the employer and will have to ( in this case ) abide by UK law.
They could not expect to go to court and say we fired the crew member because a customer company did not like them, they would have to have grounds to fire the crew member or the courts would award compensation to the employee.
In terms of employment protection it matters not who foots the compensation bill but that a crew member has the leagal protection afforded by UK law.