Apply a mask to the half you want to lighten, then feather the mask - the portion that hangs over the edge of the photo will feather too, but will not matter as it is past the edge (just make sure it is larger by a sufficient enough margin).
Now you have two choices. Many times the effects you apply will only apply to the masked portions of your image, but sometimes they effect the entire document. A more sure fire method that will probably yeild more controlable results would be to make a new object from the masked portion, then do all of your lightening and brightening to the duplicate. It would be easiest to hide the original image, then make your adjustments to the duplicate. Since you won't move it from its location, when you make the original image visible again, the effect will appear to blend on the same image.
In retrospect, an easier method might be instead of feathering the mask, just make the mask with a hard edge, duplicate the masked portion, then use the Interactive Object Transparency Tool tool to manually create you fade. This will allow the added benefit of being able to fade in multiple locations/directions if needed.