A Friend in Need

How do you tell a good friend she has a problem when she doesn't even listen to you about every day things anymore?! I am concerned for her and her well-being, and being a friend I truly believe that I should point this out to her, but her personality is one of being extremely defensive and I fear the end of the friendship if I try and talk to her. One may argue that maybe, if she's not really willing to listen, then maybe she's not a true friend at all, but part of me feels like that is just an easy way out of trying to really be a concerned friend. Kind of like the proverbial, "screw it!" Or one can also say that she's an adult and can live her life as she chooses, and should be mature enough to accept to consequences of her actions if that's the road she wants to take. *Sigh* It's just really difficult to watch from the sidelines and do nothing. Lately, things have been getting progressively worse, and I'm sure that pretty soon she'll start to question my difference in behavior around her because I just don't know how to be a friend around her without speaking up.