This is why the 12-hour clock system, with its inherent ambiguities mustn't be used when noting the time actually matters. If someone's going to use the 12-hour notation they should use the verbose "noon" and "midnight" terms instead of "12".

Even worse, this is ambiguous as well. When is "midnight" on a given day, is that the earlier or the later time?

Best-practice is to specify a non-midnight time, i.e. "must be turned in by 23:59 Friday evening"

I get your point, but your example should have been "must be turned in by 23:59" or "must be turned in by 11:59 Friday evening".

If you're going to use both, I might as well say "midnight Friday evening," or even the worse "12:00 Friday evening" as both of those are still straightforward.