How awful for him @Tracey248 . Has he already spoken to the guest about her behaviour and followed it up with Airbnb so there was a record?
This will help Airbnb review the case.
He should call them about the situation and ask Airbnb to cancel the booking and get advice from them about packing up her belongings. He may need to bring them to the hospital if he doesn't feel safe. If she is able to leave hopefully the hospital can help the guest identify mental health support/access to emergency beds.