As in “She’s such an airhead, she’s always losing her keys”. I don’t want to be insulting. I’m looking for more of a cutesy word to make her smile.
“Peanuthead”?
Just say that her brain runs on sailboat fuel.
Dingbat. Hey, it worked for Archie Bunker…
Sarah Palin?
(too soon for a GQ joke?)
now, seriously: Bimbo and Ditz aren’t terribly offensive.
“She’s such a Blonde”
Ditz.
Is ditz really less offensive?
“She’s so absent-minded” seems like a good way to go.
Well, daydreamer might be the least offensive, although ditz came to mind pretty quickly.
There’s no need to sanitize the comment to the point of meaninglessness - it’s obviously a friend that he’s trying to be cutesy with.
and yes, I think (but that’s only me obvs.) that ditzy is less offensive, primarily because it just sounds cutesy.
On the contrary, Peanuthead indicates a highly intelligent, sophisticated, handsome, wealthy, suave, honest, all around cool dude.* 
*Who just might not always tell the truth. 
Yeah, I kinda like “ditz”. Cute and accurate without being insulting. But keep 'em coming anyway. She might like a little variety. 
Well, there’s “space cadet”, but that’s probably about even with “airhead”.
Pneumo-psychic.
Birdbrain.
Ditz.
Flibbertigibbet? Will-O-The-Wisp? Clown?
knucklehead… turrrible. just turrible.
Flighty
Another shot at it:
“She’s so synapse-challenged”
I thought flighty was used to indicate more tentativeness, rather than forgetfulness?