Seems she spawns when Canyon Queen is killed and Vice a versa
This is not correct, killed Canyon Queen twice earlier and the Hollows Widow didn't spawn either time. Also the Canyon Queen spawn is triggered by the quest for it, not any ph.
Also needed for green breath quest (and have heard is needed for all of them). Note, a long spawn time so it is hard to get the kill... Tends to spawn near the entry to the spore cave (southern most of the four caves off the Hollows) and then wander about the area of the bear cubs, hollows drakes, etc. until killed...
She is in the field to the left of the entrance area - the part that is bordered on three sides by the river. Seems to be a random spawn in the area. Has a very nasty poison attack for her level.
She is required for the first breath weapon quest - definitely for the bloodline of Atathus the Red Lord, and based on other comments I've heard for at least one other bloodline as well, if not all of them.
Correct - also required for the Black breath weapon. Was yellow to level 4 when I encountered it, a passing lvl10 was happy to help me whack it, took about 10 seconds.