Paranoia and delusions

Hi everyone, I'm new here.

i have been with my partner for over 4 years and have learnt that his Mother has suffered with anxiety in the past. Recently she has been extremely paranoid, she won't leave the house if no one else is home as she believes people are entering the house while no one is there. The house has security cameras and my partner showed her a weeks worth of footage to prove that no one has come into the house, but she still won't believe him. She does her shopping when my partner and his dad get home from work, she won't speak to her daughter over the phone or via text message as she thinks people will target her daughter. A few times whilst my partner & his dad were at work and I was at the house, she asked me where I was going and if I was going to stay for the day because she was worried about being alone. She seems to go through episodes of this every now and then, I know that she is on medication but I'm not sure what it is. But it seems like she stops taking it and then things get worse. Her husband tried to get her to see a psychologist but she refused as she does not want to leave the house. It is affecting her whole family & relationships with family members.