I'm not really certain your friend's assertion actually requires a refutation, since line art is exactly the sort thing that Adobe Illustrator and other vector-based graphics programs do best. You certainly wouldn't want to make a comic like this using MS Paint or even high-end pixel-based software like Photoshop. At least, not if you care about the quality of your product at all.
All your friend really did is prove that he has no idea what he is talking about.