While your student can start looking into different schools as early as they want to, the fall semester of their junior year is when their research should begin in earnest.
Along with comparing degree curriculums between schools, students should look at sports, clubs, and extracurricular organizations they want their school to have and start compiling a list of schools that attract their attention.
They can begin by looking up videos that have been posted by current and former students or through the school’s official channels. Websites like College Navigator or the College Board can provide good insight, as can informal commentary that they might find from their peers on sites like Reddit or social media.