I have some familiarity with UIUC, and I’d say, if the qualifications listed in the OP are accurate/representative, this student is very likely to get in (getting in to the College of Engineering would be less clear, but Mathematics is in LAS at UIUC). I also agree that this student is very likely to get into Case Western. (Yes, I know, that and a few bucks will get you a cup of coffee …)
That said, it’s still a good idea to consider other “safe” schools. For STEM-oriented students in Illinois, some possibilities are Purdue, Wisconsin, Minnesota, Iowa State, MUST, and Rose-Hulman.
Regarding safeties, the general advice here is that they’re the most important schools on the list, or put another way, build your list from the bottom up. I agree that safeties are very important, but my take is a little different. I’d say that matches are the most important – in that they’re probably the largest group on the list, and that the most time will be spent considering them. But there are a couple of corollaries that are very important here – that you go into the process being able to assess things fairly objectively, reasonably, and realistically, and that you plan for various contingencies, even some fairly unlikely ones. And that will still necessitate including a few “safe” schools.
So, OP, I think you need to add a few matches and maybe a couple of safeties.
Another thing regarding safeties, I vaguely recall that when I first started visiting this site a few years ago, people would list four criteria for determining a safety, not three (as in comment #27). I can’t remember what the fourth one was, maybe something like the school had to have an acceptable major available? (Or maybe my memory is just going ).