It's a sad day for the seela, but not the end of the world. In adventure 3, when the heroes make it to Seaquen, there will have to be some adjustments, but there are a few people at Lyceum that will take her place in imparting story information, so the search for Etinifi can be conducted by the heroes at a later time and someone can teach the 'Song of Forms' if needed (Gilver Fern).
Tiljann's place in the story is to provide exposition and plot hooks that the heroes can pursue. If she dies, there are other ways around the issues. In fact, Tiljann goes offstage after adventure 3 and can optionally reappear to remind the heroes about Etinifi in Adventure 8 (where he appears), but Gilver Fern could provide the info.
It is assumed she is still alive in adventure 12, but her role is a support one that can be performed by any Wayfarer.
