She should talk to her OB about this. Women who start a pregnancy obese are at higher risk for complication and can benefit from loosing weight during pregnancy. This should be done under the monitoring of a physician.
If your friend had a terrible diet with excess calories and lack of exercise until now, changing her diet to a healthier intake and reducing calories to a normal level can result in weight loss without having to starve herself. Pregnant women do not actually need a lot of additional calories, it is more important to take in normal levels of healthy nutrition (protein, minerals, vitamins) than "eating for two". Pregnancy is not an excuse to "pig out" - and eating too much and gaining too much weight is usually more dangerous than gaining too little, especially if starting out overweight.
Also there have been numerous studies on the benefit of moderate exercise during pregnancy. So while it may not be a good time to start training for her first marathon, walking, swimming, yoga and moderate cardio can be very beneficial, if ok'd by her doctor!
For most women weight loss during pregnancy equals less gain of weight. It is recommended that woman who weigh more gain less which can result in a net weight loss during pregnancy.
Pre-pregnancy weight: Recommended weight gain
Underweight (BMI less than 18.5) :28 to 40 pounds (about 13 to 18 kilograms)
Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds (about 11 to 16 kilograms)
Overweight (BMI 25 to 29.9): 15 to 25 pounds (about 7 to 11 kilograms)
Obese (BMI 30 or more): 11 to 20 pounds (about 5 to 9 kilograms)
So if you really want to help your friend, tell her to talk to her doctor or midwife about her diet/eating and exercise habits. I had a midwife for my pregnancy and she had me do a food and activity diary for two weeks in my first trimester as part of my regular prenatal care to discuss appropriate nutrition and fitness during pregnancy - but I don't think many OB's do this routinely...
Good luck.