Layers man, layers. Yeah man. OK, layers, like in the pic above, with the original, the picture of the hot chick is the bottom layer...the picture of the friend's wife's head is another layer on top of the hot chick. So, erasing everything around the wife's head will remove her body and the background of her picture, while leaving the hot chick background intact.
I don't know much about GIMP, but I'm pretty sure it can do this...if it's worth a poop it can, and I've heard it's worth a poop or two. I use Photoshop though.