I don't want to write a letter.

A co-worker of mine who just graduated from their credential program last year did not get her contract renewed. When she was freaking out, I frantically told her I would write her a letter of recommendation. She emailed me last night cc'ing admin asking if I would deliver on writing a letter.

But in retrospect, I don't actually want to write a letter. She was a terrible teacher who set our kids back in math skills by a mile and triggered tension and friction in our department. I have nothing good to say.

What do I do?