In season #1, Mickey was representing an accused murderer in a high profile case. The individual who really committed the murder gets himself signed up as Mickey’s client at his office on another matter, then tells him that he committed the murder his innocent client is accused of.
This has actually happened at least once. At very least, a lawyer in this situation must withdraw from representing the murder client, because he cannot defend him zealously if he is prevented from revealing evidence that would prove that client innocent. He can’t reveal it because that information is a confidential communication from another client. I hold that a lawyer in that situation must withdraw from both representations, because he is likely to hold such animus against the guilty client that he is unlikely to do his best job representing him. (Hating your client’s guts is a serious conflict of interest.)
Mickey, clever dog that he is, resolves the problem by tricking his murderer client into revealing his crime to the police. The scumbag’s arrested, and his innocent client has the charges dismissed against him. But a lawyer can’t do that. He can’t use a client confidence to the detriment of the client or for the benefit of himself or a third party, in this instance, the innocent client.
I know what the writers and even some lawyers might say: Mickey’s solution serves the interests of justice. The bad guy got caught, and the innocent mad was exonerated. But if clients can’t trust their own lawyers not to sabotage them because it just seems like the right thing to do, then clients can’t trust lawyers. At very least, what Mickey did to solve his problem deserved a long suspension, if not disbarment.
Another little detail is that only a lawyer can form an attorney-client relationship. Mickey’s office can’t “sign up a client” all by itself. Apparently he needs some brushing up on Rule 5.3, which holds lawyers bound to educate their non-lawyer staffs regarding legal ethics.
Ethics Alarms 
Did the actual murderer contract that Lawyer’s firm in bad faith to ethically trap the lawyer? Would such a scenario change the ethical duties involved?
Yeah he did, but had a colorable need for representation. His motive wouldn’t let the LL off the hook.
If only the lawyer can form client attorney relationship then if the admitted murderer got his timing wrong with engaging the lawyer, the lawyer could present that testimony and subpoena the man?
A local bank is the single biggest client of the law firm where I work, but bankruptcies are the most frequent type of case we handle for individuals, so the office manager and I are always checking the Documents folders on our computers to see if a defendant the bank wants us to proceed against was a bankruptcy client of ours. (Likewise, if we’re already representing the bank in a collection or foreclosure against a would-be client, the attorneys will tell them that we can’t represent the would-be client, whether as defense counsel in the collection or foreclosure, or as their bankruptcy attorney.) I’ve also overheard our attorneys telling clients in other types of cases that no, they can NOT represent both sides, no matter how well the first side to contact us gets along with the other side. Basic legal ethics and common sense!
Did they even address the conflict in that episode?
I don’t remember that episode but that is a surprisingly bad story (overall, I like the Lincoln Lawyer and can disregard the unrealistic legal elements). You could not get halfway through reading the standard rule on conflicts of interest without concluding this is not permitted. It’s not even plausible. Yet plausibility or “close calls” are what make for more interesting drama because it can reveal the character’s character. Here, he just looks like a bad lawyer who either did not know the rule, or simply decided to ignore it.
-Jut