Normally when I am asked to write a reference for someone who was poor I simply state:
<<Name>> worked at X from <<date>> to <<date>>. His/Her
role was that of <<position>> and had responsibility for
<<responsibilities>>. He/she led a team of <<number>>.
If the person was good, I'll state it explicitly. I expect the new potential employer to read from what I didn't say when it comes to someone that was not particularly good.
Does anyone else do the same?