Cute card.  
 
I would definitely tell you can't do Barbie, but say you'd be glad to modify any details of the card to fit her ideas, such as colors or adding something like a doll instead of the cupcake. I'm sure you get the idea.

Obviously when I suggested about adding a doll to the Princess instead of cupcake I meant a generic doll and not Barbie.

A lot of internet shoppers who frequent sites like Etsy probably do not understand copyright infringement. If you've ever gone through Etsy you will find Barbie's, cartoons, and more all over.  
 
The site Sandra mentioned looks great (as does her card). They are "doll" like and maybe would suffice.
 
Is she going for a Barbie look or the Disney Princess characters? Both would be a copyright infringement but they are two different looks. The Disney ones would still be cartoon but change the dress. The Barbie would be more doll looking but she may also want the Disney style dress.
 
I don't know to me it would involve way too much effort for the sale BUT I would look into how you could make a more recognizable princess without infringement for future sales.
 
Good luck and be sure to post what you come up with. Your designs are adorable as are.