Actually, both exercise and being vegetarian will probably help her--but, as others have pointed out, miscarriages are WAY more common than most people know (which I think is too bad--it's always a tragedy when it happens, but only after I had mine did I learn that SO MANY women I know have been through the same thing and we just don't talk about it--which is too bad!). I'm now 36 weeks pregnant with my second child--had one miscarriage between the two, at 9 wks. I was not vegetarian for my first pregnancy, but am now, and am having WAY fewer complications--my OB attributes this to my diet change; vegetarian diets (done well) are usually much healthier, and healthier is better for having a successful pregnancy. You get plenty of folic acid from fruits and veggies, and can get a lot of iron, too (my sister, who is not vegetarian, is currently pregnant and anemic--I'm not). As for exercise, I asked my OB about whether I should stop yoga for my first trimester--as my instructor recommended--and he said that I should continue whatever exercise I was doing before, and it would NOT cause a miscarriage. He said my yoga studio was probably concerned about liability issues--that I would blame them if I had a miscarriage--but my dr. was clear that continuing the exercise I was already doing would help, not hurt--there is always a 30%-40% chance of miscarriage, but the healthier and in better shape you are, the better the whole thing is likely to work.
I'm very sorry for you and your daughter's loss, and will pray for your family as you get through this. Even knowing the risks (I'm 36 years old, first pregnancy was high risk, and tried again knowing that things might not go well), I was caught off guard by how devastated I was. But your daughter will get through, and hopefully try again.