My friend has a son with down syndrome and a newborn. how should she explain it to the younger one when he is old enough?

Honestly: Children are amazing and intuitive and pick up differences long before you are aware they have. Simple and honest explanations work best. Answer the question the child asks...Understand it at his/her level and then you will know what to say. It won't be hard. I have several families in your situation and the siblings get it. They are ind and protective and see the beauty of their sibling.

Love them equally: If you show love for both children equally, they will accept each other equally. As special problems surface, they can be discussed and dealt with in an appropriate manner. If other children or adults make comments, you can explain how other people can lack understanding and they need to be accepted for their weaknesses.

Be very honest: Your friend might consider describing the child with Down Syndrome as a slow learner. Later, depending on the level of understanding of the younger child, additional information can be given about the etiology of the intellectual issues and what problems will come up later in life. It is not necessary to give too much information at one time.