This wire designed is suitable for dual gas applications, 100% CO2 and 80% Ar+ 20% CO2.
I attached the EN standard (classification) in my earlier email, which can bring his understanding.
He use 100% CO2 gas, so the classification of EN standard C1 means 100% CO2 gas.
To use correct shielding gas is certain, client shall not refuse to accept it.
These are the evidence for them to believe on what you said to them.