As a vegetarian I say good on her! It really isn't that hard. Once you start cooking some vege kind of meals you realise it is a lot cheaper than eating meat and she can definitely be healthy on a vege diet. First make it clear to her that for her to choose a vege lifestyle is a responsibility on her part to make sure she eats a well balanced diet. You can't just take meat out of a meat eaters diet and expect it to work.
Just as a basic she needs to eat a wide variety of vegetables of different colours. It is a common myth that vegetarians lack protein. Protein is in pretty much every vegetable. The general rule is that if you consume enough calories (and eat your daily portions of vegetables) then you are eating enough protein. The things she really needs to look out for are vitamin B12. If she doesn't have enough of this it will cause big problems. B12 can be found in fermented soy products, soy milk (as an additive though), eggs, dairy, some seaweed and some mushrooms. If she doesn't get enough from food she needs to take a vit. The other is iron. Iron is in lots of foods including tofu (soy) and green leafy vegetables. Having vit C at the same time will help absorb it. As for her bowel problems, a vegetarian diet can actually help a lot with that as a good vege diet has a lot of fibre in it. Also forgot omega 3 - linseed is good for that.

As for subsitutes, there are lots of meat substitutes (e.g. vege sausages, vege patties) and depending on why she wants to be vege will affect whether she likes these things or not. If she doesn't like the taste then she probably won't like them either. If it is for ethical reasons then she might. Either way, consider them to be a bit like junk food - just have them occasionally.
Common meals that we have include:
1. Stirfry, with lots of vegetables and steamed rice
2. Nachos/tacos, instead of mince in the mix use beans.
3. Vietnamese fun rolls (rice paper rolls that you wrap up salad and meat/marinaded tofu)
4. Satay and rice, with vegetables (if she is underweight nuts and seeds are great!)
5. Salads with nuts and avocado
6. Vegetable curries, especially using lentils and pulses - cheap easy and so nutritious
7. Roasts, lots of roast vegetables and vegetarian gravy - yum!
8. Soups, pumpkin, vegetable, lentil etc.
9. Sushi - with any vegetarian filling (I like Fry's vegetarian nuggets) and miso soup (source of B12)
10. Pizza - top with lots of vegetables, and try grilled eggplant, or artichoke, macadamia nuts are also good.

Check out some vegetarian recipe websites for more ideas. Stick with the aussie sites though as I have found the American ones often use products we can't get here.
Hope this helps!