LOL your friend can’t “change” his major (called a concentration at Harvard) yet b/c he doesn’t have one. He wasn’t admitted as a history major- he was admitted as a general student, same as every other Harvard applicant. Concentrations are normally officially declared at the start of 2nd year, though it can happen earlier. Your friend will be assigned a history advisor as their first advisor, and will presumably connect with the bio department when he goes to declare (the department has to accept you as a student- it’s essentially a formality, but it goes better if the student has actually talked to somebody in the department.
That’s because, as @happy1 said: Harvard does not admit by major- you can apply ‘undecided’ and get in. Your friend might think that he was very clever to lie, but irl there was no benefit to him, and a possible disadvantage (if course availability is limited it goes first to the declared majors, then to the prospective majors and then to the general student population, usually starting with seniors).
Of course, that’s if indeed he was lying! It is possible to love two things. Your friend may just think that Bio is a more sensible choice, especially if he is thinking pre-med (he’s actually wrong- there are too many bio majors out there- but that’s another story), but part of his heart really does belong to history.
At most LACs and the school of Arts & Science at most universities* you don’t have to declare your major until the end of second year. If you want CS at UC-B, getting into history and then transferring is hard- arguably harder than getting into CS in the first place, but at other universities / for other subjects it is do-able.
*there are exceptions