Originally Posted by meph
Tell him that you're unwilling to write a dishonest letter of recommendation, and that it's probably better, for his sake, that you don't write the letter. He should get the clue, especially if he knew he was going to be fired.
This is the best option, I think.
Just tell him honestly that you don't think you would be comfortable writing an entirely positive letter of rec -- you would have good things to say, but would feel compelled to include unflattering facts, and he might be better off without a letter of rec.
This happened to me with a professor once -- I had a good rapport with him and he was one of my favorite profs, but I had a bad habit of dropping/flunking his classes. I asked him if he would be willing to write me a letter of rec, since I felt he knew me best of the faculty and knew the extent of my intellectual abilities. He said he would be happy to, but would be obligated to include reference to my flakiness and poor work ethic, and that it might be more harmful than helpful.
I felt that was a fair response, and got a rec from someone whose classes I had aced instead, even though I didn't know them particularly well.