Judging from "fascinated, don't you worry, will we see", is it possible that she liked him before? What do you mean by butterfly days ? One's prime time or riding high days?
Hearts and Hands by O. HenryThe girl's eyes, fascinated, went back, widening a little, to rest upon the glittering handcuffs."Don't you worry about them, miss," said the other man. "All marshals handcuff themselves to their prisoners to keep them from getting away. Mr. Easton knows his business.""Will we see you again soon in Washington?" asked the girl."Not soon, I think," said Easton. "My butterfly days are over, I fear."
It's not a common expression. Perhaps they shared some time in the past, perhaps a day spent chasing butterflies.