Totally depends on what he's going to use them for. If they are of his plane so he can hang them on his refrigerator, I'd let him have copies at cost or very close to it like the previous post said, and you can can be proud he likes them. If he is going to give them to his boss to make brochures for his airline, I'd charge a pretty penny. I think use is the main determining factor.
Since you said files, I'd let him download them for free off airliners.net for personal use only. If he wants bigger/better files, then you'd better determine what his use will be, and warn him that any other use than personal use would be copyright infringement.