SO VERY sorry for your loss. SINCERE condolences...
My late father in law, a very healthy and spry 87 years old, doctors thought suffered from a severe stroke and was not rehabilitating for about 6 weeks.
He began a sudden decline and doctors found an undiagnosed brain tumor. Within 9 days we lost him. It was not a stroke but the undiagnosed brain tumor from the onset.
Hope that you are blessed with family and friends for support.