With pregnancy depending on her starting weight they may want her to gain 20-30 pounds with total pregnancy. Anything more can be harder to loose after birth of baby or may cause complications during birth. She may want to check with her dr to make sure her wait gain is in the right track.
She is lucky I am 22 weeks and have gained 15 lbs. The last thing you should be worried about is her weight she is carrying another person she might gain another 20. If she gains to much the doctor will say something.