What majors are general type of major is he considering?
If those majors are particularly competitive for undeclared students to get into at one campus but not the other, that may be a reason to choose the campus where the competition for those majors is less intense.
UCI: https://changeofmajor.uci.edu/
UCSD: https://students.ucsd.edu/academics/advising/majors-minors/capped-majors.html
UCSB: apparently no central listing; check web site for each department
In terms of “good liberal arts foundation”, courses for that can be chosen at any of them, but it may be more required if the general education requirements of that type are more extensive (e.g. if he got into Revelle at UCSD).