Yes she can exercise. She should not exercise to the point of raising her body temp dramatically. She shouldn't lie flat on her tummy or on her back. She should keep her head elevated slightly if she's on her back. For cardio: walking is great, elliptical machine is great, dancing is great. Gentle resistance training and stretching will help with labor. I'm a nurse and certified trainer. I hope this helps but please make sure to check with her doctor.
Yes. Especially if she has been exercising all along. The main thing is to not get her core temp too high. When that happens its like she has a fever which is bad. She should look into exercises for pregnant women or ask her doctor. But walking is definitely perfect.