Most knowledgeable employers will check more than one reference. Also, if someone has worked for a long period of time for one employer, and then suddenly is let go for "poor performance" that's sort of a red flag that perhaps there was more to the story and most smart prospective employers will not totally take the word of another employer as gospel. And that is what I'd say when asked about my situation with the past employer (hypothetically, of course!) I'd say, "Well, I worked there for xxxx and my work seemed to be suiting them fine, and then I had a few issues(all resolved now) for which I had to be away from the job for a short time, and when I came back I was unexpectedly terminated for performance issues. I had had no idea there was any problem. I always do my best on the job, as you can see if you contact xxxx company, where I worked prior to this."
And whatever you do, do not make huge issues out of your reason for termination, do not start going into what he said, what you said, how much conduit you used or didn't use...etc. You're just cutting your own throat if you do that.
This sort of thing is perfectly legal. Of course this guy was just culling you because you had had an issue, and he was looking for someone to get rid of. Therefore, his bogus opinion of you isn't really important in the course of things. If you were terminated in this hypothetical situation, you'd file for unemployment quickly (performance issues are very hard to prove, especially with no write ups or warnings) and you'd very likely be approved.
Many times, these sorts of companies know the reputations of other companies in the same field. They may not be willing to take the word of this company and may realize that they tend to give bad references.

