Sometimes it helps just to be able to talk about our problems. If your loved one with SAD ever takes the time to open up, be sure to listen carefully to what she has to say. Don't judge what she says or tell her that she is being irrational. People with SAD know that their fears don't make sense but they continue to have them anyway.
In addition, spending some time listening to your loved one talk about her fears will help you better understand the challenges that she faces on a daily basis. Once you know which situations are the most difficult, what she is feeling in those situations, and what she wishes you would do to help, you will be in a much better place to offer assistance.