A friend of mine told me that you can pretty much eat whatever you want within about 15 minutes of a big exercise session without putting on weight. I can't really understand the science behind this. Is there any truth to this?
This is not true. Your body will process the food that you ingest, and if the caloric intake of the food is greater than the amount of calories burned during the ensuing exercise, you will gain weight.
You can have some carbohydrates before your workout if you find that your diet doesn't supply you with enough energy to not feel weak or excessively tired during your workout. You can also add protein before your workout if you find yourself incredibly hungry in the middle of your exercise. Again though, if you take in more than you burn, you will gain weight.
I understand that there is a window (not sure if it's 15 minutes exactly, I suspect longer, and there's probably a gradual drop-off) after a heavy strength-training session in which your body is in a hormonal state that causes it to route most e.g. carbohydrates into repair and refueling your muscles & supporting systems, rather than routing it into fat storage.
Keep in mind as well, however, that digestion takes time and certainly more than 15 minutes so you might be better off doing that eating before or during your workout (not to the point of being stuffed) or immediately afterward.
The information is sort of correct, except it's supposed to be roughly a 2 hour window of increased metabolic rate after doing exercise that is primarily anaerobic. However, this doesn't mean you can eat anything you want, in fact almost the opposite - the benefit comes from eating a small snack during this period and enjoying the 'free' increased calorie burn.
There was a recent study suggesting that just the opposite is true. Generally exercise causes short term appetite increases. This study's results showed that if you ate something before exercise, your brain doesn't compensate for those calories as much as it should, so you end up eating more overall.
No, this is false. Let me provide an example. You can't eat 100 Oreos before your workout and expect no weight gain or negative side effects. Regardless of whether you are working out or not, if you eat a lot of junk, you will get fat unless you are genetically gifted of one of our unnatural friends.