Depends upon on the University since some schools do not take into consideration your alternate major during the original admission process. UCLA does not consider alternate majors. If you are waitlisted and admitted, you should be admitted to your first choice major which rarely happens so most likely, said student will not get off the waitlist.
If the University is like UCI which admits into the University first and you are waitlisted for your 1st choice major which is highly selective/impacted, chances will be the student will admitted into the alternate major or Undeclared.