If she has B’s and a C in high school classes maybe she isn’t targeting the tippy top colleges? If that’s the case she’s probably fine with the level 3 language and moving to AP music theory if she wants to. All my kids were accepted to excellent colleges and even were awarded competitive full tuition scholarships with three years (or less) of foreign language. They all chose more music/art/theatre over another year of language.
Caveats:
One of my kids (two years of Spanish in HS) had to take additional foreign language in college (essentially starting back at level one because she’d forgotten it all) to meet her college’s core requirements. Part of the way she accomplished this was by doing her college’s summer language program in Argentina. If your kid quits language now, you could be paying for a trip to South America later. 
You really need to check the colleges your student will likely apply to, because requirements differ and most colleges do have an expectation that the student will have some level of foreign language coursework in high school, or meet a language requirement while in college, or both.