Yes I’ve noticed that at some schools, MechE and Aero are in the same department, or have really substantial overlap, and at some schools it’s also pretty easy to double major. I’m not seeing as much overlap in the curricula at CU and I agree that it’s also not obvious whether there’s a lot of social overlap.
I would suggest that she look into some specific student clubs of interest, and contact them directly (by email or whatever contact method they give on their club web sites). My son is really into FSAE, and when he was evaluating colleges, he found that most FSAE clubs were very responsive and happy to talk to him and meet him (and to show him around when he visited). These informal club visits turned out to be the best thing he did to learn about being an engineering student at each school.