There is nothing wrong with asking, in fact that is the polite thing to do on this website, since so much work is stolen. Not sure why anyone would consider it inappropriate for someone to ask. The only thing that I could even see wrong with him asking about specific art pieces, are those that were commissioned. Since those are paid for pieces of art, one should expect to have to pay to use them in their own work, unless the person who paid for them is ok letting them be used.
It falls under the specific user thats been asked to determine if its rude behavior or not, though I could never consider it rude cause its actually a compliment for someone to ask such a thing, even if you turn them down.