Is there any way to get a schizoprenic person in denial diagnosed?

Interventions are used for getting the attention of rational people. They do not work with psychiatric problems. The only options for a person that has a psychiatric problem are for someone to keep pressuring her to see a doctor, let it go, or seek a court order to have her declared mentally unfit. Court orders are not granted unless there is compelling evidence of a threat such as a history of police interventions, emergency room visits, or commission of crimes. Your grandma is free to live her life as she pleases even if you know that she would be better off with some type of treatment.