Call the airline and ask what's the likelihood of her flight being diverted; where might it be diverted; and what is their policy, if a 12-year-old is stranded in a strange city? At least you can tell your SD that there's a plan and what to expect, in a worst-case scenario.
I don't think her concerns are at all immature. My step-son has been flying unaccompanied between California and Indiana (min. 6 hrs.), 6-8 times/year, since he turned 7. We HATE it. We couldn't afford to accompany him (TWO adult round-trip tix for every one round-trip ticket of his...). His mother, who decided to move out there, can't afford to accompany him, either.
While DSS was under 12, in order to put him on a plane, you had to sign a form acknowledging that the airline takes no responsibility for him whatsoever; that if he winds up in a strange city or the person picking him up isn't there early, waiting for him, he will either be shipped back to the "sender" - at their expense - or turned over to CPS in the city where he winds up, whichever is more convenient for the airline. Now, in actuality, they DID pay attention to who was supposed to pick him up, and checked their i.d.; they sat him in the front of the plane so the flight attendants could fuss over him; he could only take direct flights; and the person dropping him off had to remain by the gate until his plane got in the air, so that in case the flight were cancelled at the last minute, somebody would be there, to be responsible for him.
But he just made his first flight since turning 12 (the cutoff age for the Unaccompanied Minor program, at least on Southwest). It was as though he was an adult! He was just another passenger, to them. They did not care who picked him up. Even though I emphasized that only my husband was allowed to pick him up, in California, DH's ex-wife got there a few minutes earlier and the airline let her go back there to pick him up anyway. She doesn't have the same last name he does. They did not even check who was allowed to take him. He's allowed to change planes, now, and if his mother schedules him on such a flight, he's expected to change planes entirely by himself. I do not understand on what planet it can be considered safe for a 12-year-old child to potentially get off one airplane, figure out for themselves where/when they're supposed to catch the next one and hang out for who knows how long in a huge, busy airport hundreds of miles from anyone who knows them. When I dropped him off, my 3-year-old was having a complete meltdown, so I gave my cell # to the attendant and explained I couldn't stay until the plane got off the ground, but if something happened, to call me and I'd turn right back around to come get DSS. The guy pointed out that DSS was 12 now. He just did not care about having any way to reach me. So, a week before, when DSS was 11, Southwest thought someone should be around to take him home, if the plane didn't get off the ground. But now that he's 12, he's just supposed to find a cab and get home on his own?
I've heard people point out that you never hear about kids getting lost while flying alone. I have heard of one case - I think it was last Christmas, in Denver, when a flight was diverted due to bad weather. Maybe other cases don't get publicized. And maybe the parents who do send their kids unaccompanied are mostly super-anal-retentive about the travel, because - especially past the age of 12 - the airlines sure aren't.
I note from a prior response that she is on a direct flight and, evidently, an airline that still considers 12 an UM. That's good. I think she still deserves an answer, though, as to what would happen if her flight were diverted? And, if she's going to keep flying like this as she ages out of the UM program, I think you guys need to keep her on direct flights and explore what happens if a kid's flight is diverted when they're over the UM cut-off age. I don't know. Technically, the airlines do not take responsibility for them. But they're too young to get themselves a hotel room. So, what DO they do?