Educate yourself on the problem by reading about the subject on the internet, in magazines or books. But a little reading doesn't make you an expert and may even be resented by your friend. She may get angry or upset if she feels you are cornering her. It is important that you are ready for that reaction. If the problem is big enough for you to notice, she may know about it, too, on some level-but expect her to deny it.
Denial is a powerful and common defense.
If she attacks you, let her vent. Don't respond back with emotions or make it personal. Keep the issue at hand. A denial reaction may be her protection from facing the real conflict. Sonnee suggests setting limits.