Yes, let's blame this on Chaz's disguise skills rather than Nina's lack of attention while carrying packages. That's a great idea.
"Oh no-- I'm so sorry--!" As the moment of shock from literally running into a person fades and she's certain she won't fall again, she moves to take back the packages before looking at the stranger and sets her froyo on a nearby table. "-- You're not hurt, are you--?" Yes, the quill comment was probably a joke, but it sounds like something that could actually happen.
Then she sees the stranger's face and does two takes before she's blinking at her unfortunate victim. It's not like Nina knows his face that well, hasn't seen it in person since she was a child, but sunglasses don't exactly hide the face she's seen on a poster or two at school-- or the family resemblance, in the little details of his face, even if on the whole he doesn't exactly share the Warrington aesthetic found in the rest of the siblings.
After a few moments of silence, because frankly, she'd have expected to run into literally anyone else, "Chester! I mean-- Chaz. You're Chaz Warrington, right?"
Yes, that is exactly how one greets their estranged brother.