man, no way have you put on 12kgs in one week, unless you've been eating concrete, you'd doing well to put that on by the end on a good cycle
I can put on 5lbs in one day with a carb load, I dont think 5lbs in a week is out the question by any means. Had he said he put on 5lbs of muscle I would agree but 5lbs of bodyweight is relatively easy to do.