Was not Britain Orthodox before it was Roman - I'm thinking of the Celtic Church. I'm new to BOC, and open to correction, but if I am right this might give you a little ammunition.
Furthermore, the 'English Church' (as distinct from the Roman of Augustine) is a relatively recent development from the sixteenth century, whereas we (from the Celtic Church) may perhaps claim much greater antiquity here.
Of course it also raises the question of when did this country really become British or English - a field day for the historians! Were we Celtic before we were British/English?
In any case, surely Christianity is not native to Britain. It was imported from the Holy Land by one route or another, so how does your friend feel about that I wonder? Presumably a religion with a native ethos would be something like druidism or other non-Christian form of worship.
I rather fancy that there is an element of mischief in such discussions, which tries to get an admission that one form of Christianity is better or more authentic than another, a pretty unhelpful and pointless argument.
With love and prayers,