The best bet, I think, is to notify the primary care physician that you are seeing changes. Write things down, document them (dates, specifics) and what's been said/done by family members (if anything) and whether that worked or didn't.
The doctor can't talk to you without the patient's permission, but can and will take in the info you send. Tests can be conducted, medication can be adjusted or considered, and recommendations can be made.
I did it with my mother, and my good friend just did it with her mother, which resulted in her mother going into assisted living. The doctor called the patient in (under the pretext of checking blood pressure and so on), and used the family-provided info to conduct additional tests (neurological, mental sharpness, balance, etc.) and tell her that he had determined that she just couldn't live alone anymore. (It had been discussed at prior visits.) The family also included in the letter what the living options were: she can have an apartment in our house (and she tried it for a month), she can alternate among her 3 children, she can sell her house and go to assisted living, she has no support from siblings, has rejected home health aides, etc. Whatever you have to offer that is relevant.
If the person is a danger to self or others, there are services that can be mandated.
95% of the time, you can't accomplish this on your own. People don't want to hear that they need physical help even if they are mentally sharp, and even less so when their needs are psychological. An objective professional that the person trusts is often the best option.