If your friend doesnt has questions about his case, he should ask those questions of his attorney. The attorney is obligated to answer those questions. In the end, the choice of whether to go to trial or take a plea is your friend's and his alone - NO ONE can make him say he is guilty if he doesnt want to.
Most criminal cases in the US are resolved by guilty pleas or dismissals. Typically, the plea offer made by the state is better than the possible result of being convicted after a trial. The prosecutor, in theory, makes an offer based on what s/he thinks is fair, based on the evidence the police have collected and whatever information the defense attorney has presented. It may be that in this situation, the state feels it has a strong case that your friend was involved in at least one count of arson. (Burning garbage sounds more like reckless burning than arson.) Only your friend can decide if a plea bargain is better than going to trial.