Should a Fan-Fiction Writer Be Able to Judge an Original Writer?

Now before I start this I want it to be understood that I am not trying to get down on fan-fiction writers or belittle their writing, I've read amazing fan-fictions and I've even written a few myself. However; this is more in relation to a situation a friend of mine is in because of a fan-fiction writer. Now he claims this writer is his best friend and that she's a great writer, but she only writes fan-fiction, she doesn't write anything original and my problem with this is, she completely downs his writing. He wants to be writer some day and I'll admit he has things to work on, mainly just spelling and grammar, but I have grammar problems too so it's not like I'm judging him.

He is an amazing writer, he wrote his own original story created a world with history, cultures and different concepts all on his own and because of her he doesn't even believe that his writing is good. He was under the impression that all of his characters had the same personalities because of her and was genuinely surprised when I told him that none of his characters sounded the same and that I could tell them apart easily.

She has told him his writing is not good and she doesn't believe that he will ever be a writer, but and I may be wrong for thinking so, but who is she to judge his writing when she has yet write one single thing of her own? I mean I'll give her props that her fan-fictions aren't cringe turners like a lot I've read, but with a fan-fiction you have plots, characters and concepts already made that you can lean on, when you're making an original you have yo research and and come up with all of that stuff by yourself so how can you even think about downing his writing when you don't even write your own stories?

I think what pisses me off the most about this situation isn't even that she's a fan-fiction writer casting judgement on an original writers work, it's that she's supposed to be his best friend, but instead of trying to help him become a better writer, she's just tearing him down instead.