UCD is not a safety for a student with a UC-weighted GPA of 4.19 applying to a popular major like mechanical engineering. UCs tend not to emphasize SAT/ACT scores as much as courses and grades. The second tier GPA range of 3.80-4.19 had a 52% admission rate last year; the top tier GPA range of 4.20+ had an 89% admission rate last year. With a 4.19 GPA, one might expect that the admission rate would be between those percentages (but obviously not anywhere close to 100% that would make UCD a safety), but then applying to a popular major pushes the chance down.
Remember also that subjective criteria like essays are significant as well.
In the other threads about UCD, most of the “odd” rejections are of applicants with second tier GPAs applying to popular majors. Most posters seem to be overestimating the value of high SAT/ACT scores, when UCs tend to emphasize them less.
Note that students who apply as some other major and then try to change into a popular major after enrolling may find it very difficult to do so in terms of needing a high college GPA and/or a competitive admission process to get into the major.
He would have been better off adding additional less selective schools where he can be admitted to his major as actual safeties (e.g. all UCs except UCSC have mechanical engineering, as do 13 of the CSUs). Of course, if he is willing to start at a community college and then transfer to a UC or CSU as a junior, that can be his alternate plan (use http://www.assist.org to plan course work).
Note that if he met either of the “top 9%” criteria for UC, he may get admission to UCM (which does have mechanical engineering) if he gets shut out of his other UCs.