Who can my friend contact sue to losing her home to the massive rain that destroyed her apt.

Best to contact FEMA. Red Cross does help. Here is their web link. https://www.redcross.org/get-help/disaster-relief-and-recovery-services.html

Not sure but can call Red cross or emergency services of Stockton

Red Cross may have disaster relief information for your friend, but this location is a blood donation site. You friend should also speak with the property manager for the apartment (and hopefully you friend has renter's insurance). the property manager is required by law to have insurance for exactly this type of problem.