I have been subpoenaed to testify in a criminal case but as it is my friend, I don't want to do this. What will happen if I don't show up or when on the stand just plead the 5th?

You have a constitutional right to refuse to testify on the grounds that your testimony may tend to incriminate you. Assuming your testimony could incriminate you (not just your friend); you should advise the prosecutor of your intended refusal before taking the stand. Generally, the prosecution should not call witnesses when they are aware the witness intends to invoke his privilege against self-incrimination. However, the prosecution has the option of seeking to compel your testimony via a grant of immunity from prosecution. If the court grants the prosecutor's request for immunity, you cannot be prosecuted for any offenses you reveal during your testimony. Thus, you no longer have a right not to testify. If you continue to refuse to testify after being immunized, you can be held in contempt of court. This may result in your being jailed and/or fined. Do not fail to show up. A warrant will be issued for your arrest. I recommend you retain the services of an experienced criminal defense attorney prior to your court appearance date, who can advise you as to whether you have a legitimate concern of self-incrimination, and if so, can engage in discussions with the prosecution on your behalf. Perhaps he or she can convince the prosecutor your testimony is not helpful to, or could jeopardize the state's case, and he or she will release you from the subpoena. If not, and you are immunized, you should have an attorney present with you at your court appearance, to provide immediate representation if you are found in contempt. These are not waters you want to navigate alone.