She may want to go to
http://www.epilogue.net if she's really, really good. Someone I used to know had a gallery there, and she got commissioned for book covers/illustrations by people who visited her Epilogue gallery.
If Epilogue is not an option, I would recommend
http://www.elfwood.com. It looks nicer than dA and because every piece of art submitted is accepted or rejected by a moderator, it's not full of crappy doodles like dA is. Another plus is that Elfwood automatically puts a copyright under each submission, with a link to a page where the copyright rules are explained. She could link a Cafepress account to her EW account to make it easier for people to buy stuff.
She could use dA if she wants to. The plus side of dA is that it automatically links the visitor to Cafepress if you've put a particular drawing up for sale, but for the rest I'm personally not too fond of dA.
She could also consider starting her own website, but only if she's certain that she would make enough money to cover domain costs and if she knows something about/wants to learn CSS and HTML.