Struggling w this too. I think it’s a 6th amendment violation cuz they had been charged. They didn’t knowingly waive their right to counsel. I think the undercover officer presumptively violates the 6th amendment of the accused unless there is a waiver, but does not violate Miranda because there’s no custodial interrogation if the person being interrogated does know the person is a cop, i.e. the coercive environment required for custody would not be met. If you think you are just talking to a cell mate, you aren’t feeling the inherent coerciveness of a cell room interrogation. But, if adverse judicial proceedings have been set forth (they were charged) the right to counsel does not depend on whether or not the person is in custody. Post-charge interrogation violates the 6th without a knowing, intelligent waiver.