This can and does get even worse.
Suppose the lawyer does breach confidence. Anything he brings forward can not be used in court. It can not even be used as basis for an investigation. But the lawyer would lose their license, and probably be sued by the client he ratted out.
Now suppose the lawyer keeps his mouth shut. But the truth somehow comes out years later. As a member of the legal process the lawyer can be held on charges (uncertain of title) for failure to disclose the info that was in his possession.
Note: I am not a lawyer nor am I totally 100% on this, but it is how I recall one of the legal paradoxes. It is the basis of the disclaimer statement "If you are guilty of anything do not admit to me directly" (or words to that effect).