Just Wish: Malorie Blackman, Whizzywig and Whizzywig Returns (London: Random House, 2005) c. 1995, and 1999.
Whizzywig is an alien who takes refuge in Ben's bedroom while she fixes her ship, and in second book comes down to Earth to make research notes. Whizzywig grants wishes but they must be for other people. It gets a bit confusing because this turns into "for people other than Ben" which also seems to mean that if other people make wishes for themselves (Aunt Dottie wants to be able to play the piano) that's do-able. As you can tell, I thought it all rather sloppy and really simply a magical morality tale about thinking before you speak. I had the feeling that Blackman chose an alien because boys won't read about fairies.