Before you do anything else, contact the SF Rent Board or a rental attorney. Only they can give legal advise.
I have been a landlord in the past and my opinion would be that a landlord cannot be held responsible for an "Act of God". It's too bad that there are rats in the home to begin with so forget the couch and get her out of there!
If you need a referral for an attorney, I'm happy to help.
Have a great day;
CEO & SR Credit & Mortgage Consultant of
Ex-Mortgage Broker of more than 10 years