It is illegal for an attorney to inform the police if their client admits to a crime in the past, because it is a violation of privilege.
If it is not a client, they do have an obligation to inform the police.
Also, if they tell their lawyer they plan to commit a violent crime, then the lawyer must tell the police, because it is a safety issue.
However, if they put their client on the stand, knowing they intend to lie, the lawyer can lose their bar license. If the client admits to the crime, and the client wants to go on the stand an lie, the lawyer must remove himself from representing his client. This does not mean he can't allow the client to plead not guilty, but he cannot allow the client to testify to false facts.
If a lawyer violates the privileged information, it would not be admissible in court and it could easily result in them losing their bar license.
Hope this clears up your question.