Just be honest with them and let them know that you may not have the answers or understand what they are going through but they are important to you and you are there to listen.
@SolGryn I know. Friends feel helpless and can get frustrated. Just listen and tell them they are important and love. They will hear the words but it will take a lot of processing to believe them. Remind them (if this is a repeat) that they are stronger than they feel and the feelings will pass.