Unsure what to do... help! Any advice?

If APS doesn't respond, I'd try calling an ambulance. If she can't get up then she needs medical help. She is not competent to take care of herself.

They may be able to take her unwillingly due to her filthy situation, citing that she is not in her right mind and is a danger to others and herself. After a checkup at the hospital I'm sure they'd send her for a psych evaluation. There she would receive the help she needs.

GA,
May I politely disagree;
"but plan how to "assist" them so she doesn't end up on some state run warehouse for older people." 

Just my take, but I don't think that a non-relative can intervene with a neighbor, nor would APS necessarily want her help. As far as I know, they just do they checking and referring.

IMHO, I'd rather see this woman clean, well fed, given medical care, given psychiatric care, medicated and looked after in "some state run warehouse for older people" than lying on her filthy kitchen floor, covered in her own feces and urine, possible rats running around in the rotting food, smoking, unable to walk, naked and obviously out of her mind.
Unfortunately, there is no way to "control" the government system. Watch a few TV shows on other countries and what their institutions are like. Makes the U.S. look like the Hilton!

How about calling "Hoarder" TV show to see if they will help your neighbor?

After years of caregiving and eventual change to post-caregiving, my mind is becoming clearer than it has been in years. And I now see that intervention could be viewed more in the vein that the OP and posters suggest.

I have limited but negative experience with APS, which refused to intervene when a relative needed protection from her caregiver. So I don't have a lot of confidence in them, but unfortunately they're basically the "go to" for intervention when necessary. So try to turn them to your way of thinking of saving intervention instead of the often stated approach of someone refusing to take care of herself. This woman can't and deserves sympathy and compassion.

I wouldn't consider it "reporting", or "cleaning up" as I used to. I see it as a critical intervention to help someone live the rest of his or her life in a more positive and rewarding manner, and identify and learn the coping skills that can best help her.

It's easy to be critical of people in this condition, but who knows the whole story? And, speaking from experience, it's easy and I think instinctive reaction to refuse help. Who really wants to admit, especially at age 65, that she can't care for herself any longer?

And you would be doing this woman a favor by getting help for her. As you write, she's "brittle", unwell, too ill, or overwhelmed or afraid, embarrassed, too proud (?) to accept help, but she can't turn her life around w/o it.

So, yes, be her guardian angel by finding help for her, but emphasize that aspect, that you're concerned for her and want her to be able to live a rewarding (rest of her) life. If she won't accept help, APS would be the go-to agency, but plan how to "assist" them so she doesn't end up on some state run warehouse for older people.

It wouldn't hurt to plan out the whole intervention and work with APS - they might even appreciate someone stepping in to help them with their monumental task. Spend some time researching Meals on Wheels, home care assistance, osteoporosis treatment, mobility (if she can get therapy enough to walk, or do weight bearing exercises, it'll help counteract her osteoporosis).

And as others have suggested, contact her family if she has any.

I agree. This lady has some serious mental issues and probably health issues as well. You’re a good person for wanting to help her. Call APS and tell them what’s going on. You can report her anonymously but she needs immediate help. She has no family?

Your neighbour is 65. That is not old.
She isn't just a bit untidy. She has lost the ability to care for herself to the extent that her surroundings are now seriously hazardous, and she is at real risk of harm.

So, if you think you'd feel bad about calling APS - I assume because you think it's only one step removed from setting the dogs on her, which is a sadly common notion but also a wildly inaccurate one -

How do you suppose you would feel if her burned or rotten corpse were wheeled out on a gurney, and you had known all the time how much trouble she was in, and you hadn't even made a single phone call to help her?

I appreciate that people hesitate to call social workers for help. I just wish they wouldn't. What the heck, in truth, are you waiting for?

Desidumes,
Yes, call APS. Tell them what you've witnessed.

The lady must not be in her right mind to behave like this. She needs to be cared for. You will be helping her receive badly needed care.

This also is very unsanitary and poses a health risk to whoever goes in there and her neighbors.

You'll be doing the right thing.

Absolutely call APS. Also if you know of any family members that she has call them too. This woman needs somebody to intervene on her behalf. Her living conditions are a hazard to herself (and maybe others) and won't get better without help.