I don't know what kind of diet he was on to say that he gained 5 lbs overnight. That is highly highly unlikely.
For him to put on nearly 5 lbs of water weight by eating salty foods, he would have to consume A TON of salt and would definitely be seeing the effects of that. Take a look at what he ate. It's not much - at least compared to the typical American diet and portions for one meal. There is not enough salt in that dinner that he consumed to cause such weight gain - the only way there would have been is if he salted everything on his plate like it was going out of style.
There is no reason for his diet to cause this, and there is no reason for him to alter his diet any more or to change the way he exercises. Trying to "flush" things out is not a good idea because it can lead to more weight loss. This yo-yo dieting effect is actually more detremental than being above your ideal body weight, but healthy because you exercise on a regular basis.
If the original poster is reading this now, continue with whatever you were doing before you ate this meal, disregard anything that the scale told you. There is no possible way you could have gained 5lbs overnight. Even if it were water weight, it will go away in time. Don't do anything drastic to try to get rid of it.