Following Sandy Parshall's suggestion, I have put sugar water and a halved tangerine in the greenhouse near the butterfly.
As of Saturday afternoon, it was still alive.
The greenhouse is chilly, and the butterfly has not attempted to fly. But I've set it near food and water, for what it's worth. A brief life . . .
UPDATE: Sunday morning and poor butterfly is gone. As are the two tangerine halves. Somewhere mice are feasting.