Fit in Physics or 4th year of Social Studies

No

This is difficult to answer without knowing 2 things - her classes as a fresh/soph, and examples of the colleges she is targeting.

In terms of academic rigor, IB diploma pretty much trumps everything else and colleges will generally allow leeway on their HS course “recommendations” when school/district/state requirements come into play.

In general, for top colleges anyway, social studies is one of the core subjects where relatively few colleges have a 4 year requirement. For her career objectives, she would be better served having a HS background in physics. If she’s already a junior, however, it may be a challenge having 3 sciences senior year where 2 are HL.