He doesn't interrupt to reject him, but he doesn't say anything for the space of a few seconds after Ginsberg stops speaking. It's a more complicated decision than he probably realizes, and Ned is weighing the options in his mind. On one side of the scale is his desire not to hurt Ginsberg's feelings by turning him down; is his belief that he may as well say yes because wherever he is today, he's going to be miserable; is his hope that a distraction might even help. The other side of the scale, though, is almost as burdened, with worries that he'll be miserable and moody and only end up hurting Ginsberg more; with his impatient desire not to have to deal with new people today; with his overwhelming urge to just hang up the phone without giving any answer at all.

Eventually, and in a strained voice that is a little higher than normal, he says, "Okay." A pause, then, "I don't need to dress up, right?" If costumes are mandatory, that might be enough to change his answer.