Wouldn't a better translation be 'God blesses my friend', i.e. no article?
I have the same question. Although I guess it would depend on the faith of the person saying the sentence. In real life English I would say that this would be a correct translation but I guess that in the fictional world of A Song of Ice and Fire the majority of faiths believe in some form of polytheism so we may need a distinction between a god and the gods. Unless you are a follower of R'hllor!