Couple of things to note first - losing weight during pregnancy is not generally to be advised.
Secondly, baby is like a parasite and if she does not have a healthy diet, the baby that suck mum dry of all her stored vitamins, minerals and other nutrients, with the risk of mum becoming malnourished herself (bub will be fine).
To answer your question specifically, depending on how overweight you are, it can increase the risk of gestational diabetes, per-eclampsia and dvt. On top of this overweight ladies may not be as fit as healthy weight ladies which can make labour harder.
If your friend is only a little overweight she should not need to lose weight.
The recommended weight for healthy weight ladies is aound the 10kg mark, remember at birth babies weigh 3-3.5 kgs and added to that is the placenta, amniotic fluid, extra blood etc etc and it all adds up.
For overweight ladies the recommend you keep the weight gain to 3-5kg.
For obese ladies, those with a Bmi over 35-40, weight loss is acceptable ONLY if they eat a very balanced & healthy diet, taking pregnancy vitamins will not replace a healthy diet.
I hope this helps