Re: No defence?
Echoing what Jørgen said, sounds like a horrible experience, and I hope the guy is doing OK!
I don't really see a defence for that. Not just with Aikido, but with any martial art. If you are already that close, and the knife isn't visible, then I doubt that anyone would have fast enough a reaction time to block the attack. And not only block it, but block it in a way appropriate for a knife attack.
The average human reaction time is, what, around 0.3 seconds, right? Even if a martial artist, through training, could cut that in half, that might not be enough to completely avoid the slash. Especially if it was as fast as you said. And if it really looked like they were just being casual and doing small talk, then chances are the guy that was slashed, never saw it coming, didn't expect it at all. You'd really have to be a very skilful and highly trained Martial Artist to avoid something like that, no matter the style.