Artorias is capable of taking a lot of hit, and given the fact that he is also fire resistant, he can will have no trouble going up close and personal with Quelaag. He deals massive damage, assuming you don't move out of the way.
They are both really slow, and quite a lot of Quelaag's attacks causes her to be vulnerable to incoming damage. I believe Quelaag will be capable of taking a little less then half of his health, but when Artorias enter his rage mode, he will make short work of her.
Lorewise, I don't think there is any doubt either. Artorias was one of the four knights selected by Gwyn. According to his description, he was also unmatched with a greatsword. Now corrupted, he still a dangerous threat to be recon with.
Also, Quelaag wasn't even in her demon form during the war against the dragons.