I think that the area around USC is fine (and getting better with the new USC Village opening up); then again I came of age in New York in the 70s, so everything is relative
I personally would pick USC over CMU, but I moved to sunny So Cal from the east coast, so I’m biased.
That said, CMU may be the better choice for mechanical engineering, but I don’t really know. Either way, she can’t go wrong as they are both great schools. Is there any way for a quick visit?