Hell, how did you even knkw a bad reference was given?
I was able to find out what was said. Again, if all someone had said was "I don't like him", that wouldn't have been an issue. This person made negative, fraudulent claims about me and my past work performance in front of enough people that it was impossible for him to hide his tracks.
People who do bad things are usually awful at keeping secrets. There are exceptions, of course, but generally the traits that incline a person toward malice and petty conspiracy are not traits that make a person good at keeping secrets.
If there was ever a time for "show, don't tell"...