Well, there's the problem. It was all the way back in chapter 2 or 3, and I had forgotten that Tanya was actually important for anything because that period of the game has a ton of people who aren't, between Marty, Osian, Ronan, and honestly most of the Fanna Freeblades. I knew Dagdar was important later, but I totally blanked on the otherwise unremarkable early-game archer, and now we're finishing up chapter 6 and Eyvel's just been frozen, so it's been a fairly sizable amount of progress. It wasn't an explicitly unfair death, but he was taking risks with her that he wouldn't have if he had thought a character might lead to something later, under the impression that she was essentially disposable.
While I agree in most instances, on this one I feel I failed in my duty as a navigator to give him the ideal Thracia experience, which involves going to 8x and seeing that content.
That's how I figure it, too. I'm not much of a computer sleuth so I figured I'd ask around a bit before I go off on my own to try work it out manually, see if anyone else has figured out what the flag is for dead or alive, and save myself from trying to brute-force it with a pretty limited understanding. I'm not asking anyone to go out of their way for me, of course, but I would've felt like a real fool if I spent a long time trying to work it out only to discover it was already known somewhere.