this to Romeo:

If love be rough with you, be rough with love;
Prick love for pricking and you beat love down.

In other words, he is telling Romeo to forget about Rosaline by having sex with other women. He is, therefore, not a good friend in this instance because he is far more interested in having fun than in listening to Romeo's problems.

In addition, in Act III, Scene I, Mercutio proves himself a bad friend to Romeo by fighting with Tybalt. When Romeo tells Tybalt that he loves him and does not want to fight, Mercutio does not show any support. Instead, he calls this a "vile submission" and goads Tybalt into fighting. Moreover, when Mercutio is killed by Tybalt, he curses both Romeo and his killer. This is not the sort of behavior that we would expect from a friend.

However, looking deeper, we see that there is some genuine friendship between Romeo and Mercutio. If there was not, Romeo would not avenge his death by killing Tybalt and, therefore, risking his relationship with Juliet. Moreover, by fighting Tybalt himself, Mercutio saved Romeo from injury or even death.

Mercutio's role in the story is to act as Romeo's foil or contrast. Mercutio provides some comic relief when Romeo is brooding on Rosaline and some action when Romeo does not want to fight Tybalt. Mercutio and Romeo are, therefore, supposed to be different, and this should not detract from their clear sense of friendship.

Mercutio serves as a friend to Romeo by being a counselor, albeit a very comical one. We first meet Mercutio when he and Benvolio are trying to persuade Romeo to crash the Capulet ball and Romeo is wailing about how heartbroken he is. Mercutio's response is to say,

"If love be rough with you, be rough with love; prick love for pricking, and you beat love down" (Act I, Scene 4).

In other words, he is counseling Romeo to fight off his negative feelings, rather than give into them.

Another way that Mercutio serves as a friend to Romeo is by remaining practical and criticizing Romeo for his wild, untamed emotions. In this same scene, when Romeo continues to be downcast and to refuse to join the fun, Mercutio offers to rescue him from his low feelings, saying,

"we'll draw thee from the mire of this sir-reverence love, wherein thou stick'st up to the ears." 

We also see Mercutio tell Romeo he is speaking nonsense to worry so much, especially about his prophesying dream that something bad would happen that night, when he says,

"True, I talk of dreams, which are the children of an idle brain, begot of nothing but vain fantasy."

In saying this, Mercutio is telling Romeo that he is being idle and indulging in fantasies.

Mercutio is a normal teenager who likes to hang out with his friends, practice the art of witty verbal exchanges, and he is loyal. Mercutio disrespects authority like many teens would and he does it in front of his friends for respect and for fun. For example, he finds it fun to crash the Capulets' party with Montegues, insult a Juliet's nurse in public, and stand up to bullies like Tybalt. As a friend to Romeo, he tries to bring logical reasoning to his friend's foolish heart, but he also intervenes in the fight against Tybalt when Romeo won't fight for himself. Sadly, this costs Mercutio his own life, to which he protests in the end by saying, "A plague o' both your houses!" because if it weren't for the feud he wouldn't have died young (III.i.103). Ultimately, Mercutio gives his life for his friend and that makes him an even better friend.