My brother in law is like that. He's been diagnosed with paranoid schizophrenia, and his drivers license has been suspended after an unrelated episode 10 years ago. He's harmless, and perfectly capable of getting it unsuspended if he puts in some effort, but he can't because:

• His doctor is out to sabotage his life
• Someone is tapping into his phone
• This lady on the other side of town is stealing his mail
• His PC had been bugged

....allegedly.

He sometimes takes his meds, but it's rare. Those are the days when he's out and about and reasonably normal.

She asked me twice whether I thought her stories sounded crazy, so she was clearly somewhat aware of her condition.

Not necessarily.

It's very likely everyone in her life were telling her it's all in her head, she gets mad and says she's not crazy...

Then calls random repair people, tells them the story. And asks "am I crazy" because most businesses would never say that to a client. She was looking for validation, the same way people go fishing for compliments saying stuff like "I'm so bad at my job, I don't know how you all put up with me". Even if it's true and they're dead weight, most people will be polite and reassure them.

I just didn’t know how to deal with someone like that

Tell them that they should relay their concerns to a medical professional if they're concerned.

If they're seriously doubting their delusions, they'll go get help and thank you for the advice.

More likely they'll realize you're not giving them what they want, get mad, and often blame you for being involved in the conspiracy.

But there's a chance they actually get help.

Any kind of acceptance of their beliefs, no matter how tentative, reinforces it and drives them further into the delusion. Depending on how involved her family is, she might have called them immediately, and after cussing them out said even the plumber agrees she's not crazy.

I just didn’t know how to deal with someone like that.

You tell them the truth. That means if you think their stories sound crazy you say “I think your stories sound crazy”.

I know that steps outside of the typical path of politeness, but telling the truth is the only way to help someone in that state.

She wasn’t asking because she didn’t know. She was asking because she knew they sounded crazy, and she wanted to give you an opening to discuss that.

Trust me. When a person is having paranoid delusions only the truth can help them. Saying “No that doesn’t sound crazy to me”, if it does, only makes it worse. That’s because people can detect when others are lying to them. If that person is so far out there that everyone puts on a mask around them, it will reinforce the idea that people are shifty assholes. If nobody ever tells them the truth then they can’t calibrate their sense of what’s real and what’s not.

It may seem rude, but if you truly want to help them, you need to be truthful with them. That includes saying things that might not be polite, such as “I think that sounds crazy”. They will not interpret that as rude. They will interpret that as honest, and it will be an enormous relief to them to have found an honest person.

I know I'm replying late but did you ever contact your countries adult protective services and make them aware to get her the help she needed because them stating they want to kill someone is worrying