Hello mollis,
I feel for what you're going through as right now, hospitals won't admit someone unless he is a danger to himself or others. You will need, to the best extent you could, monitor her behavior and at the first change you can exaggerate that she is a danger to others or herself, get her to a hospital. This could require boldfaced fudging of the truth that she is a danger.
Because, already, she has crossed a line. The restaurant owner would be within his right to call the police, yet if they arrested her for a petty crime, she could be jailed and that would start a cycle of going in and out of jails instead of a hospital where she'd get the best treatment for her mental illness.
Your aunt also needs to get tested [bloodwork] to determine if there could be another medical condition causing her delusions, yet it is quite likely schizophrenia from what you've said.
When a loved one lacks awareness that he has an illness [it's a symptom of the sz, called anosognosia], I recommend the family members read Xavier Amador's book, I Am Not Sick, I Don't Need Help, which coaches the family members in how to talk to the person to influence him to seek treatment. It sounds like your aunt lacks the insight that what she thinks is happening, isn't really happening.
Like I said, you may want to greatly exaggerate the danger she is to the restaurant owner if you see it getting out of hand [though it's already gone too far]. The thing you could do that's a strong possibility is to talk to the restaurant owner, explain what is going on and offer to go with him to the police to urge themt hat your aunt needs help. The restaurant owner could lay it on thick that he doesn't feel safe and she needs to be in a hospital, not a jail.
I wish you find relief soon.
Regards,
cb