What might you do?

My sister had a root canal done couple months ago. (Just the first part when they extract the nerve) They could not do the final sealing of it because it started hurting. They said it can't be inflamed, hurting or tender to completely finish the crown part.
They believe because her diabetes has been so irregulated, it's causing the tooth not to heal.
She has two choices, have the tooth extracted or see if they can retreat it.

She's going to talk with the doctor tomorrow about the pros and cons of either choice. In the meantime what would you do?