There's no such thing as a "friend" walking into the hospital to advocate for you. If you're in the hospital and you're awake, you are the final say. If you are not awake, it's next relatives, and if not that, it's whoever has power of attorney.
Gonna disagree with this a bit. It's a customer service world these days.
They're not going to page the doctor and have him talk to a
Twitter rando patient advocate and start making care decisions without Vickie present. However, a close friend sitting next to a cognizant patient when they talk to the doctor is reasonable for the same reason it would be at an office visit: people can get overwhelmed and forget to ask questions, or forget the answers.
From there that non-related, non-PoA person can social engineer a lot of things if they don't spill their spaghetti or start Karen-screeching or get caught smoking meth with the patient in the bathroom. An "advocate" who's there a lot, seems reasonable,
seems to calm the patient down with their presence,
is the patient's eventual ride home: they can get more info than they legally should, and pull nurses aside to give info about the patient. It's soft skills. Start threatening or filming and Security's still coming.
Hearing "I'm his advocate" verbatim from a hospital visitor usually does mean you're about to step into some degree of crazy, though, in a way that hearing "I'm his neighbor" doesn't. Maybe a quarter of the power of a SovCit in word tricks and general annoyance.
Unless it's a literal patient advocate from the local supported living agency; those guys are pretty helpful. They're more focused on a safe discharge plan for their client.