what is an appeal?

An appeal is a legal pleading that asks a court of appeals (that is, a court that oversees trial courts) to take some action based on an error made by the trial court. For example, you could allege that the judge should not have let evidence in (or should have allowed you to put on certain evidence), that the police obtained evidence improperly (like searching when they did not have probable cause), that your lawyer committed malpractice causing you to be ineffectively represented, etc. If the the court of appeals agrees with the appeals after considered written arguments (and oral arguments if they are made), then the result, depending on the circumstances, can include a new sentencing hearing or a new trial.

You should contact a local criminal lawyer who handles appeals to see if your friend has any claims worth pursuing. Your friend can also file his own appeal, but having a legal professional is highly recommended, especially if he has some good claims because the procedure only allows for one shot on certain claims and you don't want to waste that shot.

Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.