I immediately thought ditz, but daydreamer’s a better suggestion for totally avoiding offence - all the others could be taken as an insult if your dippy friend is in a sensitive mood even though they’d be taken as friendly teasing in a different mood; labelling people is a tricky business.
‘Bimbo’ would pretty much always be an insult, but I expect you’re aware of that. Perhaps it has different connotations in different countries.
pneumo-cephalic?
I like Ditz.
I was under the impression “Bimbo” describes a trashier subset of slut typically found cruising bowling alleys.
Dizzy.
Yup, 'round here it isn’t a description that a friend would find terribly endearing. 
Ditz would be my best exchangeable word.
Maria?
I think an airhead is somebody who is just plain stupid. I think you want something like “scatterbrain” instead.
Muffin Head.
“special”
She has more important things to worry about than keeping track of mere car keys.
I like the Shakespearean-era insult “wantwit,” although this may have a more negative connotation.
Also “space case.” I think it’s less insulting than airhead. Space case implies someone who is lost in the clouds, but not necessarily stupid. Airhead implies their head is empty; that is, they are dumb.
My high school coach liked the term “wall-light”.
Stick a flashlight in the ear and it lights up the wall on the other side.
Dingbat turtlebrained noodlehead! You ant skulled minus-mind! You have have nothing but mush you jellybrained helium head! If you had half a mind, it would be ten times as much as I give you credit for. That echo chamber between your ears has never had a brain cell for an audience. Bobble-head! Numbskull! Pea brain! If you could clone yourself, your sum would be zero! You could get a room at a hotel with no occupancy. For free.
In Soviet Russia, blonde jokes tell you!
/5
“You’re so…preoccupied.”
Anyone else think Air assault infantry 
Declan
Bubble (head)
A magoo.