In the pilot episode of "Suits," Harvey hired Mike despite knowing that he never went to Harvard, and has no law school background. In the beginning, Harvey and Donna were the only two people who knew Mike's truth. Then, Jessica found out, and later used this secret to threaten Mike, and as a result betrayed Harvey. Mike revealed the truth to Rachel. It took a little while for Rachel to accept Mike's fraud, but the revelation enabled this couple to build a relationship based on trust.
Rachel loves Mike and in the previous episode, she decides to move-in with him. She has turned out to be his biggest support. Now when Louis is on Mike's case, Rachel meets Louis to stop him from exposing Mike. But, it looks like Louis is not in a mood to listen. In the sneak-peek of "Suits" Season 3 Episode 12, "Yesterday's Gone": Rachel can be seen saying to Louis, "we're moving in together, we're starting a life together, and I'm telling you this because I know what he did, and I still love him."
The problem is that Louis does not love Mike, as he can be seen saying to Rachel, and exposing Mike's fraud is not a difficult decision for him. But, both Mike and Louis share a common love. It is the love for the law. In the sneak peek, Rachel can be seen using this common love to build a case in favour of Mike. Mike did not do anything to Louis, but he did what he did because of his love for law. Rachel can be seen telling Louis, if he does not let it go, Mike will never be able to practice, again. Louis looks thoughtful in the sneak peek.
However, Louis is not the kind to let go things easily, and Mike's lie is not a small issues. Official Synopsis of "Suits" Season 3 Episode 12, "Yesterday's Gone": "Both Mike and Jessica lean on Harvey for support when things from their respective pasts throw them off their game."