Yep, they both work in slightly different contexts ;) If you have a class full of mean people, then it would make sense for you to say "That's Jared- he's one of the 'friendly' people in my class." If you're just going to describe someone, you might say- "Oh yea, that's Jerry- he's friendly." It's almost more like a reassurance that you would give someone if they were afraid to approach a specific person. Otherwise we don't often describe people as "friendly," or at-least I don't, haha. You'd be more likely to hear a more colloquial statement, like - "He's really cool," which basically means the same thing. I'd say it's kind of assumed that people are friendly; the negative aspects of people's personalities are usually what get mentioned. So, it might be more common to hear "he's not very friendly." Basically, we add some descriptive words to emphasize how they are out of the ordinary. Likewise, if someone is exceptionally friendly, then it makes sense to point out 'how' friendly they are. "He's one of the friendliest people in my class." So that works well for one or two people, but not really any more than that.

Both correct but the meaning is different. "He's one of the friendly people in the class" means that there are some people in the class that are friendly and some that are not. He is one of the friendly ones. He's one of the friendliest means that he is more friendly than most of the people in the class. You could also say "He is one of the most friendly people in the class".

Yes, they are both correct.

If you think he is the friendliest person you have met, you can say "friendliest" but if he is generally friendly, you can say "friendly." I hope this helps