Here is my wife's response to her,
"Joy is not a 'feeling' but a 'knowing' that Jesus loves us and endured more suffering than we can imagine so that we'll have eternal life. That eternal life does start now even though we are stuck in these sin-sick bodies. Remember Jesus is our 'cure,' and He enables us to look beyond our circumstances. I know that it is not easy, but it can be done. The 'world's' definition of joy is earthly... a "present" joy in things of here and now. What we have though is so much greater! It is eternal.Joy. It changes our perspective on everything.
It helps, too, to think of others who are facing trials that are worse than ours and somehow have found joy in it. Joni Eareckson comes to mind. Or my neighbor who has had a stroke a few years ago. She sits in her chair and prays for others. Recently my friend who has a seven year old diagnosed with cancer after losing a child and having heart surgery on another child and she says, "God is teaching me to praise Him in all things."