Bless your heart for being such a good friend.
I've been dealing with depression for about 35 years at this point, and see no likelihood that it's ever going to *not* be a part of my life (if you had told me, when I was diagnosed about 20 years ago, that it wasn't going to go away forever with treatment.....well, I sure would have not wanted to believe you).
Certainly you can suggest she mention the possibility to her doctor. If this *is* something new for her, there is a good chance treatment could change things radically. I don't want to get into medical advice here, but, from a patient's standpoint, a lot of times your condition makes you think you can't be helped, so it might be good for an outsider to point out that, yes, it *is* worth mentioning to her MD (especially if he or she is a geriatric specialist).
As a friend, I think the two kindest things you can do are 1) Don't let *her* get *you* down. Being in the company of healthy people is great for me, although sometimes I can only manage it in small doses. If you can get her to socialize with you, even if it's just watching a DVD or listening to music or going out to lunch, that's great, and try not to let the interaction focus solely on her and her feelings----if possible, try to talk about yourself, about what's going on in the world.
and, 2) have tolerance for what she can't do. I'm having a lower-than-usual period this last couple of months and have had to cancel out twice on planned social visits with friends. I'm not pleased with myself for doing that, but I'm very grateful those friends are being patient about it.
I wish her, and you, well.
ETA: just read your update. That sounds like depression, all right. Don't let a number (94) fool you. It's great that you keep going to see her. Try doing something that surprises her in a nice way----bring some flowers or a kind of ice cream she likes or something related to an interest she has. Sometimes that little brain-chemistry jolt of good surprise can divert the waters of sadness into a friendlier path for awhile.