I cannot speak for Princeton, but he should assume for HYM that the interview, if offered, is not optional.
The good news is that the interview is really informal and does not greatly impact the application. So unless he insults the interviewer, it won’t harm him.
The other point is that not all applicants are offered an interview. Which is basically a random draw and supply/demand. Yale, last year, only offered interviews if they needed more info. No idea what they are doing this year.
But bottom line, if offered an interview, he should accept.
UC is a moot point, since there are no interviews.