First, it usually takes about 3500 calories to add one pound of real weight (as opposed to fluid-retention weight). While you are likely to be consuming considerably more than your usual lunch calorie ration, it is unlikely that you are consuming more than 7000 calories at one sitting, even if you gorge.
Second, between food and beverage, we often consume a pound's weight of food at a normal meal. Unless you are consuming pure fat, this pound contains non-negligible amounts of water, fiber, and other functionally non-caloric components.
Third, if you and your friend tried to calculate the calories for the "tastes only" trip at the buffet, you would probably find they add up to more than that of an average meal. It is the same issue we all find when dealing with canapés.
(Consider this: the average buffet serving spoon will usually dish out no less than one quarter-cup, no matter what you do... so if a normal serving for a meal is a half-cup each of three to five low- to moderate-calorie items, and you are taking the smallest possible tastes of say, eight items, most of which are higher in calories (not to mention, considerably higher in sodium!) than your usual choice, you are starting out with the same number of calories, if not more, than your usual meal.)
So, all and told, your two-pound weight gain is likely to be mostly fluid retention due to the high sodium, partly extra non-digestible (or non-nutritive) materials, and partly caloric overconsumption. If this is the case, your weight will drop back down within a few ounces of its normal in two days -- three at the most. If, indeed, you have managed to consume an extra couple days' calories at the one sitting, then you will have gained "real" weight.
Many buffets put the cheap starchy stuff (like fried rice or a lot of bread) at the beginning of the line, hoping you'll fill up on that before you reach the shrimp and the steak. It always used to work for me <G>. So one strategy might be to go through the line first without taking anything at all. Find what you really want the most. Then go back through.