I've had a few cases of this in my table birds. Tried various treatments such as olive oil, massages, crop emptying, maggots, live yoghurt. The birds didn't seem overly bothered by such a massive, soft & squidgy crop. I did slaughter them within a week (and ate them).
However I did have a layer that developed sour crop, I tried everything but it didn't work. I stopped the procedure of emptying her crop as she was finding it too distressing. A few days later she keeled over & died while eating breakfast. A black, horrid smelling sludged poured out of her mouth.
If I had another layer with sour crop I'd probably treat it for a week, see how she was, if she wasn't any better and was going downhill then I would dispatch her before it got too unpleasant for both of us.