He is a comedian, and as it was stated in that link he's become famous for his offensive style, and there comes a point where its deemed inappropriate to laugh at absolutely anything.
But I'm not sure, part of me thinks that there's a difference between for example saying "Oh, he's like a kid with down's syndrome" as part of a different routine, as opposed to launching into a routine specifically aimed at people with down's syndrome, which is what the link seemed to suggest.
And it's not like the woman is an outsider, some Fox News idiot that sees a controversial episode of Mock the Week on its 23rd repeat viewing on Dave. She was a fan of him, obviously used to his type of humor, and yet this routine obviously struck a nerve. I'm undecided, as soon as we lose our ability to laugh at things then society has truly become soulless, and yet something about this makes me think there's a line he might have crossed.
Also, I like how it quotes Matthew Horn as calling Frankie Boyle "rubbish". That's a fantastic quote there