same goes for the federal arena as well...
From what I remember from same law classes I took a while ago . . .
The evidence that the mother obtained via listening in on a conversation was obtained "illegally" and it, as well as any evidence obtained thereafter which can not be shown to have been obtainable without the "tainted" evidence, is not admissable.
It makes no difference whether the person obtaining the evidence is acting as an officer of the state. The theory being that the state could quite easily circumvent the 4th Ammendment by instigating an illegal act (B&E for example) to obtain evidence instead of obtaining a search warrent.
You example of a partner who decides to rat out does not fit in the above scenario as he was privvy to the conversation / the criminal act and can decide, of his own free will, to make public what he knows.
In the case in question, the judges have decided "correctly" and applied the law as written and intended. The DA should have known better than to use such evidence in trial (and the trial judge should have disallowed it!!). The re-trail will most likely let the perp go free - but this is a mistake of the DA and he should be held accountable.