I selected a portion of properly exposed hair, copied to a new layer and made a duplicate layer. One to be used for left and other for the right portion of blown out area.
Changed layer blending mode to darken and used the transform tools to flip and shape the hair.
Added a layer mask to smooth out the edges and a curves adjustment to adjust color and contrast.
Finally added a PWL layer to further blend color with surrounding hair.



forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=27968799
original image by Evan: