Death is ultimate. Everyone has to die sooner or later. No one can extend his/her life. It is God's will and now your pup can't come back. You should now say such sympathetic words to make her feel better.
Coming to your second question, you should advice her by giving her examples. Make her realize the importance of every pup. Tell her if she will feed only one then the other will die soon. She needs to maintain a balance. Everyone needs care and attention. You can give her the example of a plant. If you do not give the plant appropriate light and water then he will die soon. I hope she would pay attention to your advice.
Coming to your second question, you should advice her by giving her examples. Make her realize the importance of every pup. Tell her if she will feed only one then the other will die soon. She needs to maintain a balance. Everyone needs care and attention. You can give her the example of a plant. If you do not give the plant appropriate light and water then he will die soon. I hope she would pay attention to your advice.