Law discussion post

Depending on if a defendant is charged, you learned that it could be a violation of the 5th or 6th amendment if law enforcement places an undercover officer or informant in a position to overhear a statement.  Answer the following questions:

Why do you think it matters about incrimination if a person is formally charged or not? Cite any relevant case law or statutes or amendments, if necessary.
Do you think if a cell mate comes forward to law enforcement, after hearing the suspect make an admission and without any involvement by law enforcement, the statement would be admissible or inadmissible? Why or why not?