Twice
It was bad enough being abandoned once, but she once could understand. It wasn't fair to expect Elphaba to stay, to submit to the Wizard. And really, Glinda had chosen to stay behind. How could she be mad at Elphaba for leaving when she was mad at herself for not going with her?
Twice was once too many. Especially since this time there was no offer for Glinda to come too. Especially since Elphaba wasn't just leaving her, but taking her fiancé as well. For the second time Glinda found her world turned upside-down. Twice was unforgivable.
"Use her sister."