People who talk badly about someone they don't personally know well

Gosh I hate having long titles because it just looks ugly, but I figured that nothing else could really do the justice of what I feel like writing about today. Today, I experienced a very personal encounter where a friend of whom I hadn't spoken to for a long time somewhat bad-mouthed a very special person to me. Now, this friend of mine who I hadn't spoken to for a long time is one who is very logical and often doesn't judge anyone or anything unless they have reason to think a certain way. That's why, when they asked about my recent events I had expected them to rejoice in the changes that I've gone through and the people that I had become closer to. However what I had forgotten was that he was still Joe's friend since they were young, and by that, I mean that they are very close. Before hearing my side, he had already formed assumptions of what he thought my special one would be like, and because of one joke aimed at Joe (a really stupid joke that has no truth in its basis) he seems to have created a bad image of my boyfriend. If there's one thing that I absolutely can't stand, it's when a friend talks badly about someone I am close to, and no matter how much I try to convince them that they're wrong they are just too stubborn to change the image that they have conjured of a person that they only know the name and face of. Really, what is up with that? It's no different from judging a person on Youtube based on one comment they made and their username; or as my special one said, judging a person by the songs on their playlist. Okay, so I'll admit that some jokes are a bit much but if it's one that's so obviously said to provoke anger (ie trolling) then you should just get over it. I'm especially upset because of who is being judged, and who he is being judged by.