She let her go.
We went outside for the morning and my daughter's weird relationship with a dying moth seemed to be on track for Greek tragedy as they played with like two giddy lovers this morning. They took a walk. They had a wheelbarrow ride together. They looked at bricks (not everyone's choice for sharing excitement, admittedly). They splashed in a puddle. They sat and ate an apple together. They played for a good two hours in the sand, on the swings and with the toy cart. Inseparable.
Not long after this - while I was actually fixing the lawnmower (no nipple problems this time) she brought Buttercup over. Who now had about 1/8th of her wings left. I casually pointed out Buttercup should probably go home now - like Mr's Toad does. And my daughter just said, "oh - sure" and ran off into the lilies. Without any problem at she just let her go and has been silent and happy ever since.