If she's really serious about getting preggers then she should be asking those questions of her OB/GYN or PCP. A smart girl who plans in advance would get the works and be thoroughly checked out top, bottom and in between just to be sure her health is 100% BEFORE getting in the family way.
Vitamins don't make you fat; excess calories make you fat. Excess nutrients make expensive urine.
If she really is planning on pregnancy and without seeing a doctor first, then tell her to AT LEAST take a good prenatal multivitamin AND 1 mg of additional folic acid daily starting immediately.
I'm not up for answering the exercise question, but I've always heard you can do anything you were doing before you got pregnant, maybe just easing back on really heavy lifting and abdominal work. Truthfully, your body really sort of dictates what you can and can't do when you're pregnant. I could keep up with my walking, but any kind of bending or extreme exertion caused my blood pressure to drop and I'd get lightheaded and my vision would grey out.