Hi Miss inexperienced, This is a really tough situation. Being best friends for such a long time sounds really great but from what you think try spending some time alone together talk things out but don't use the word depression to her. See how she feels what she really wants and if it ends up being something really hard to answer try to explain it in the easiest way possible to help her out her feelings and you can be like "maybe talking to someone about this help this matter" but don't mention the word therapist. Since your saying you can't help her sounds like being a really tough situation you can tell her "maybe talking to someone about this help this matter". Think about this before telling her anything.