I don’t know the full facts of this case just as I’m sure many others commenting don’t, but I do sense an element of Ryanair-bashing for the sake of it.
Having been to Hahn as pax it seems really not that bad an option in the circumstances. It’s only “in the middle of nowhere” in the context of it being about 90 mins from Frankfurt. There are towns and small cities quite close by, and crucially hospitals.
The thing is, it’s all well and good to mention Metz and Saarbrucken as alternatives but I doubt they have a glut of spare rooms either, on top of that FR have no crew, maintenance and support. Yes I’m sure it is convenient for the airline, but on balance it also seems a nearby and practical option for the crew under difficult circumstances.