Over time, even the strongest Christians will falter if they continually surround themselves with companions who are pressuring them to sin. That's not my opinion, that's what God said in I Corinthians 15:33 - "Do not be deceived: 'Evil company corrupts good habits.'" Don't fool yourself into thinking that you're so strong that you'll never give in. About the time you think that you're strong enough that you'll never give in to peer pressure is the time that Satan is licking his lips because he knows he has got you! If we don't separate ourselves from peers who are continually encouraging us to sin, then sooner or later we will give in and transgress God's will.
One New Testament example of this is seen in Pontius Pilate as he gave in to the cries of the mob. They wanted Jesus of Nazareth crucified, and what exactly did Pilate do? He gave the authority for Jesus Christ, the Son of God, to be crucified. But why did he do it? Did he deliver Jesus to be crucified because he thought Jesus was deserving of death? Absolutely not! In fact, according to what the New Testament reveals about Pilate, he knew: (1) that the reason why the Jews wanted Christ killed was because of jealousy and (2) that Jesus was an innocent man and had done nothing worthy of death (Matt. 27:11ff). Why then did Pilate not stand up for what he knew was right? Why didn't he defend Jesus with the authority that he had in the Roman government? The answer is peer pressure. He allowed public opinion to override his own personal convictions about morality. No doubt he wanted to keep the peace and avoid stirring up any unnecessary controversy; he wanted to please the people though he should have been more concerned about the pursuit of justice. Pilate really condemned himself all the more when he washed his hands publicly and declared his innocence. The washing of his hands certainly did not make him guiltless--just the opposite really. It made him that much guiltier for acknowledging what was right yet not standing up for it.
Peer pressure is a powerful thing, as Pilate found out. He was encouraged to do that which was wrong, and he gave in to it. He gave in to the Jewish mob and the pressure they exerted on him. What Pilate did was wrong; he sinned. When we heed the encouragement of our peers to do that which is wrong, we too commit sin. Those who are wise will do their best to stay clear of sinful influences--even those evil influences that come from those who claim to be friends.