Chicago Manual of Style, 14th edition, to start.
Merriam-Webster’s Dictionary of English Usage (1989): “The notion that that should not be used to refer to persons is without foundation; such use is entirely standard.” It traces the interchangeability back centuries and deems both correct in standard English.
Garner’s Modern English Usage (5th Edition, 2022): Classifies the restriction against “that” for people as a “skunked term” (a debated usage in flux), but affirms both are valid, with “who” more formal.
Easy rule: “That” is used with objects, “who” is used with human(s). There are exceptions, but this one is correct most of the time.