Our resident good girl and main character, Elena, cherished her friends. If someone was lucky enough to be included in her core group, then Elena would do literally anything to help them. At times, she would easily sacrifice others to protect her friends.
However, even good girl Elena had times when she was a less than stellar friend. When these times happened, her friends seemed to instantly forgive her. This makes us understand why Katherine, the doppelganger was so jealous of Elena and Elena's life.
Good Friend: Asking Stefan to Save Matt
When Elena and Matt drive off the bridge and end up at the bottom of the water, they both are drowning. Elena tries to keep Matt awake, but it seems that he's drifting off faster than her. Stefan comes to rescue, but for some reason, he can only save one. Naturally, he goes to save her. Elena points at Matt, refusing to be saved.
Although Stefan loves Elena and doesn't want her to die, he honors his wishes and saves her friend instead. Luckily, she does come back as a vampire since she had Damon's blood in her system, but, at that time, they didn't know that. When she tells him to take Matt, she's accepting her death.
Not a Good Friend: Kept What Happened to Vicki a Secret
Vicki, Matt's sister, was presented as an important character, perhaps part of the core group. She dated Tyler (who becomes part of the core group), and she also was with Jeremy (Elena's little brother). Damon treated Vicki like a toy. He seemed to mess with her life because he was bored, nothing more than that.
She's killed, made a vampire, and killed again. They act as if Vicki can't be saved, that becoming a vampire means that she is now evil. However, other vampires were trained. We don't know why this would have been impossible for Vicki. The consequences of Vicki's death led to two things: 1.) Elena kept the information secret from Matt; 2.) She had Jeremy's memories of that night compelled away. Both don't make her stand out in a good way.
Good Friend: Kept Bonnie's Gift a Secret
When Bonnie first discovers that she's a witch, she can think of only one friend she wants to tell—Elena. Bonnie shows Elena her magic by making feathers float—at first, then a few, and then a formation. Initially, Elena doubted Bonnie's magic, but then she is shown it. She is in awe of her friend's gift and then delighted.
It tells us something important that Bonnie felt close enough to Elena that she would be the first person she would show her talent to. Additionally, she trusted Elena, and Elena kept it quiet. Initially.
Not a Good Friend: Doesn't Realize Stefan is Gone
When Elena and Damon begin to hook up, Stefan is actually locked in a safe, left to drown repeatedly for months. True, she had thought that it was Silas (Stefan's doppelganger) in there. She also thought that he was taking some time away. However, she knew Stefan and would have known that this was outside of his character. Since she was happy with Damon, it could be that she didn't want to ask any questions and have her happiness interrupted.
Good Friend: Saves Caroline
Elena saves Caroline two important times. When Caroline is newly turned into a vampire, Caroline can't control the urges and kills a person. Her face smeared with blood, she's confused about what she's become. In this chaos, Damon decides it's best to end her. He tries to stack her, but Stefan and Elena throw themselves in front of Caroline, shielding her. At this point, Elena is human, and still willing to use her fragile life to protect her supernatural friend's life.
At another point, Alaric (briefly made evil), tortures Caroline. Elena saves her, telling Caroline to get help. When Elena was very human, she was also very brave when it came to her friends.
Not a Good Friend: Didn't Notice Bonnie Died
Out of all of Elena's friends, Bonnie is probably the closet. She does count on Bonnie perhaps a little too much for her magic, but the two friends really do cherish and love each other. It's clear that they would do nearly anything for each other. That is what Bonnie does in Season 5. As a crying Elena says, "She gave up her life so that I could have my brother back, and I didn't even know about it."
The whole summer Elena hadn't been able to get ahold of Bonnie. However, since she was so enamored with Damon and their relationship, she didn't really think much about it.
Good Friend: Believes in Stefan Even When He Doesn't
When Stefan transforms into a Ripper (a.k.a. the worst part of Stefan) or is compelled by Klaus to hurt her, Elena believes that Stefan can overcome anything. She is not the girlfriend or friend who disappears once things get tough. Rather, she finds out resources that she can use to help.
She contacts Lexi, Stefan's deceased best friend, to learn how to break Stefan of his Ripper phase. She puts herself in harm's way because she believes in Stefan.
Not a Good Friend: Doesn't Hold Damon Accountable
Elena's good judgment really swerves when it comes to Damon. While Damon becomes a better person, he can be pretty horrible. An example of this is how he used Caroline in Season 1. He fed on her, used her for information, threatened her, and had sex with her. For all of this, she was compelled, rather than making an active decision to take part in any of it. When Caroline becomes a vampire, all of those memories come back.
It's understandable why Caroline doesn't care for Damon. Elena doesn't seem to understand this. For a character who is praised for her empathy, she has little empathy in this situation.
Good Friend: Comforts Her Friends in Her Good-bye
Elena has been put in some sort of coma by Kai (one of the least redeemable villains) in Season 6. Kai linked her life with Bonnie's, meaning that while Bonnie is alive, Elena is this sleeping spell/coma. When Bonnie dies, Elena will awaken. It's like a very twisted Sleepy Beauty.
Her friends, Bonnie and Caroline, enter her head to say good-bye. As they are there, Elena spends the time telling them how much they mean to her and comforting them. When Bonnie starts to break down about how sorry she is, Elena stops her, saying, "you have spent your entire life making sacrifices for me. Now, it's my turn to do it for you."
Not a Good Friend: Turns Her Humanity Off and Hurts Friends
Perhaps Vampire Elena wanted to practice her inner mean girl (of which we know she has it) or emulate Katherine because since she isn't able to deal with grief, she turns it off. This leads to her doing subtle torture like stealing Caroline's prom dress (she knows that Caroline is a planner) and feeding on Matt. At least, she became mean-girl friends with Rebekah for a time. Otherwise, Elena without her humanity isn't an Elena we want to see, nor do any of her friends.