I had the opposite argument with one Indian guy when I was living in the UK.
He was saying what people speak in countries like Mexico, Argentina, etc. cannot be called Spanish because they are not from Spain and instead they speak Mexican, Argentinian...
I told him what they speak in those countries is still Spanish the same way that what they speak in the US is English even when they are not in England. He replied that what they speak in America is not English either but American instead.
Then I realized how stubborn he was in his wrongness and just gave up.