I’m currently a college junior pursuing a BS in CS and math, with minors in music and dance (the former of which I am on scholarship for). When I started my freshman year I was planning on triple majoring and getting a BA in math, music, and CS. I quickly realized that the few extra courses I would have to take for the music major (that I wouldn’t have taken otherwise) weren’t worth my time; my scholarship does require me to at least minor in music, so I still had to take a couple music courses that I wasn’t interested in. The ability to fit in courses also depends on the school. At my university double majors and/or minors are very common and degree requirements are flexible enough that they’re quite doable with careful planning.
My university also recommended freshmen ease into the workload with a lighter first semester, but I didn’t do that. I got nearly straight A’s while taking primarily sophomore level courses (linear algebra, data structures, etc). There hasn’t really been a difference in terms of success between me and my friends who started with an easier course load. If she wants to be ambitious and graduate in four years (like what I’m doing), she’ll need to start with a heavy course load her first semester. She can always drop a class if need be (she should pay attention to the dates, since there are cutoffs for when you can drop without penalty).
Also, I’m assuming that the music major is a BA, yes? If so, it’ll very likely be less time consuming than what many of the folks here are thinking (my brother has a BM from a conservatory and my university only offers a BA, so I’m very aware of the differences in the curriculums). BA programs typically have much less intense performance requirements (typically 1 hour lesson per week plus one ensemble per semester) and are much more academic focused (musicology, theory, etc). The one caveat is that if the engineering degree is a BS (which most are), she may depending on the university, have two sets of core courses/gen eds to fulfill, which may cause some difficulties.