when it was my mother who was delusional - I contacted one of her friends to please come over to visit her. The friend came over, and was able to convince her to go to the hospital emergency. At that time, she was admitted and finally diagnosed - she was psychotic (paranoid, delusional, hallucinating).
Contact the doctor if you know who her doctor is and explain the problem.
You could try to contact your local Mental Health Association.
You could try to contact your local Schizophrenia Association for ideas too.
When I was there with her, I wondered the same thing - how to get help. When I phoned Mobile Crisis, they said to phone the police if necessary.
I'm sure that it is very frightening for the delusional person to have these thoughts and experiences, and they are not thinking clearly. Then their behaviours are based on their irrational thoughts.