What does harm is people feeding into their delusions, it makes them more real. You don't have to be a cop and counteract everything but you can't be telling her she's right either....best to be as neutral as possible and just listen. If she asks your opinion tell her the truth but don't offer an opinion unless she directly asks.

Sometimes I do reality checks, you know---did you hear or see that because I want to know if it was real or not.