Couple things to stay away from:
Herbal Teas (some of them can make her sick during pregnancy)
Alcohol (obviously)
Large amounts of Fish (Mercury)
Her diet should be healthy:
Animal proteins, lots of green veggies, fruit, whole grains, healthy fats etc...
During the 2nd trimester she should increase her calories by 300 per day.
Proteins, Healthy fats from dairy and eggs, peanut butter, cheese, salmon once a week, and avocados, and carbs such as fruit, beans, hummus and whole grains. Eat slow and smaller meals. Make sure she is drinking plenty of water and getting enough minerals in her vitamins. An iron supplement is good to add here.
During the 3rd trimester, add another 100-200 calories per day of the same foods above. Again, eat slow, drink plenty of water and have smaller meals. Keep up with the vits and minerals and don't forget the iron.