IMO, “D” comes after “C,” not in lieu of “C.” While not saying that one can’t succeed in E&M without mech, at pretty much every college, Physics 1 (or however it’s numbered) is the prereq for Physics 2, and for good reason.
No college will ding him for not taking a course that is not offered. What they will do id view his transcript based upon what the HS offers, and how the GC rates the overall schedule. If you are asking if I personally think that an applicant targeting T20 colleges with a senior year schedule of “AP French, Calc C, Honors English, Honors US (purposefully not choosing APUSH), and two non-academic electives.” plus AP Physics C mechanics has a “most demanding” schedule, then I would say “no.” Not for lack of E&M, but for lack of humanities AP and inclusion of 2 non-academic electives. Personally I would eliminate one elective to make room for physics and choose the AP version of either English or USH. But I don’t know the context of the HS, so that’s a GC call.