Private Investigation and Taping or Recording Telephone Calls

A question has been posed to me by a private client (not an attorney) regarding the legality of secretly recording a conversation with another person. In this case, the other person has approached my client on a number of occasions and requested that my client "commit a crime" in exchange for payment. My client wants to record his conversations with this person. Can anyone advise as to what state's laws permit for a conversation to be recorded secretly?
The below site will show you which states permit recording with only one party consent. http://www.rcfp.org/taping/quick.html If you are working with and at the direction of the police investigating a crime, soliciting one to commit a crime is a crime- it's called entrapment, otherwise it is permissible to record a conversation and it would be evidence in court.