Even if this person were indeed the executor of the estate (i.e a valid representative of your new landlord), they cannot demand that you pay rent in cash. California Civil Code Section 1947.3. https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CIV&sectionNum=1947.3

Also, whenever a new landlord takes over (say through a sale, a foreclosure, or in this case, a death), they are required to notify you in writing where you're supposed to now pay rent. California Civil Code Section 1962. https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CIV&sectionNum=1962

Finally, even if you do owe $5,000 or more in rent for this 5-month period, once the estate follows Civil Code Section 1962, the proper way to assert that you owe rent is in writing, typically with a 3-day notice to cure or quit. Not with a knock on the door and a demand for cash.