Sounds like something that should be discussed at the outset of the trip. OP's friend could have said, "I don't plan to tske many pics. Do you mind sharing yours?"
I must admit, though, that to me, under those circumstances (frirndship, traveling together, etc.) an answer of "no" or "only the ones that aren't the best" would leave me pretty cold. Am I supposed to offer to purchase the good ones?
To me, it isn't like a haircut, legal advice, etc. Photos can be and are easily shared. I get the art aspect, but is art supposed to be hoarded by the artist? I know that the answer is yes for some, and of course that is their right, but I would be very put off by this position from a good friend.