Okay, everybody ready for a long post? Good.
BJCM- I never had the same problem myself, but actually the other way around. After I went a few months without eating meat and then ate some again, I was on the john for several very unpleasant hours.
But, if you're making a lifestyle change, it is very likely that you'll have some adverse reactions. This does NOT, does not, does not mean that it is bad to stop eating meat. Saying that you'll be deprived of valuable nutrients is ONLY something people who are ignorant of vegetarianism say. I'm by no means a doctor, but I'd say the diarhhea is most likely caused by something else, or it is your body detoxing. You can tell your fiance that unless you have been buying certified free range meat, you have been consuming many harmful and addictive antibiotics, growth hormones, synthetic flavor enhancers and other toxins. Your body could be expelling these things now that you've stopped consuming them.
As far as nutrient deprivation- there is not an American alive who is in danger of malnutrition, vegetarian or not. In fact, 99% of the problems Americans have with their health is caused by TOO MUCH nutrition. When you stop eating meat, you will be getting less iron, B12, and Omega 3 fatty acids, but that's it. Iron needs are easily satisfied by increasing your intake of leafy greens, and B12 and Omega 3 you can take once a week in supplements.
I smoked when I became a vegetarian, but once I had made my life that much healthier, it was much easier to quit. I also had IBS before I became a vegetarian, but now it's like, "IBS? What IBS?"
Now, protein. MEAT IS NOT THE ONLY FOOD THAT CONTAINS PROTEIN. In fact, almost everything you eat does. Peas, beans, and grains are excellent sources of protein when eaten together, and soy and dairy when eaten alone. Mock meats like tempeh, tofu, and seitan are also excellent sources but are not necessary to eat in addition to "normal" foods.
Several million Americans are some type of vegetarian, but try and rack your brain for the last case of protein deficiency reported...now rack it for the last case of high blood pressure and heart disease. Yeah, protein is really not something you EVER need to worry about.
Jasonsgirl, as long as you eat any type of animal flesh, you are not a vegetarian. A vegan eats no animal products whatsoever, including eggs, dairy products, honey, carmine, etc. A vegetarian eats eggs and dairy products. Even if you do not want to give up all meat, it is still very beneficial to your health to eliminate most of it. You don't get to have a special name, but you're healthier. 
