I need help with this. and the answer has something to do with immunity

Ben and Eric have been friends for years. One day Ben asks Eric to participate in an armed robbery that Ben has been plan- ning. Eric agrees. Ben confides he has robbed several conven- ience stores recently and has never been caught. Police arrive during the robbery and both men are caught. The prosecutor wants to try Ben for his previous criminal acts but needs Eric's testimony. How would you advise the prosecutor to assure, Eric's testifying to convict Ben