“Oh, how lucky you are.”- Who is the speaker here? Who is “you” referred to? Why is the person spoken to being called “lucky”?

The girl is the speaker of the line mentioned above.

Here “you” refers to the blind narrator on the journey expressed her love for the hills especially in October. The narrator calling on his memory replied that October is indeed the best time to visit Mussorie in October. The narrator calling on his memory replied that October is indeed the best time to visit Mussorie because by that time the hills are covered with wild dahlias, the sun becomes pleasant and at night, one can sit front of a log fire and drink a little brandy. Since most tourists leave the hills by that time, the roads get quite and almost deserted. The girl considered the narrator lucky as he was visiting hills at the most appropriate time whereas she did not have an opportunity for such a rendezvous with the hills.