Law states that the person can legally carry their instrument aboard if there's room at the time of boarding, regardless of size of the instrument case. Since she didn't board, I'm hoping this goes to court to strengthen this law as a musician.
That said, reports seem to miss the part where she bought the discount ticket that doesn't allow for carry-on luggage. Now, the events that transpired after this was discussed at the gate were inexcusable. There's no call for that, and were I in that situation I'd probably be awaiting arraignment for popping a gate agent in mouth. On the other hand, if you have an instrument described to be "worth more than her car", don't cheap out on your ticket, and sure as hell don't buy a ticket that disallows carry-ons, hoping they'll make an exception when you get there. Get the pre-boarding or whatever needed to ensure you're not last on the plane.
Personally, I pay a little extra on the ticket such that I'm not in the last group to board. I use a case with backpack straps because I theorize it looks less like an instrument, and more like I'm just some bro with his pack. Most of the time I carry a cheaper instrument ($500 or less) that's a long way from being worth what my car is, so if it goes in the hold, I won't be too upset. And if I bring the nice instrument that's worth several thousands of dollars, it rides in an $800 case just in case I'm force to check it (if it gets hurt in that case, it's because a bomb went off in the cargo hold). All that being said, I've always just walked on with the instrument and no one so much as bats an eye.
> <answer mainstream operating system: Windows/Linux/OS X>
Then they will tell you "that's not a real time OS, thus you can't possibly be doing real time music with it. Musicians can't stand like audio/keypress latency."