Are Vegetarian and Vegan Diets Bad for You?

I have a good friend who is fanatic about good health and she says not only is the vegan diet somewhat unhealthy because of lack of meat (I know, I hate the things they do to animals also) but that it can also cause severe psychological problems, sickness, lowered immune systems, and unless you are taking 500 nutritional pills a day you will probably die of malnutrition if nothing else gets you first. Are you vegan for the diet or for the animals? If its only for the diet, do you really get the best results on that one? Have you tried any other diets? Also, my mom, and 3 of my friends went vegetarian and they started really gaining weight. Why is that? Are you not supposed to lose weight on a vegetarian diet? I was just wondering what benefits there are to these two diets? And on either of these diets do you take or have to take vitamins?