Suicide almost always leaves a host of painful emotions in its wake–and one of those is guilt among those left behind. Don't be surprised you feel this way; almost anyone in your position would wonder what they might have done.
And to be honest, sometimes those guilty feelings are justified. Experts have told me that people who are contemplating suicide often send out "signals" about their intentions–signals that are really cries for help. Sadly, those signals may go unheeded. We get too wrapped up in our own concerns, for example, or we may be afraid of getting involved. This isn't always true, however, and in any case you weren't solely responsible for what your friend did.
Guilt is like a heavy burden–and the solution is to let Christ take it away. God knows your situation but He still loves you, and He wants you to give this burden to Him. Jesus' invitation is for you: "Come to me, all you who are weary and burdened, and I will give you rest" (Matthew 11:28). Ask Christ to come into your life today.
Then ask God to help you be more sensitive to those around you. You may never face this exact situation again–but all around you are people who are searching for peace and hope, and God wants to use you to touch their lives for Christ.