If the mental breakdown was due to something the accused did, such as excessive alcohol or drug use, there will be no benefit to him: the law does not let people benefit from their own wrongful or reckless behaviors, or from conditions which they cause.If it's due to some mental illness or trauma, then it may result in a reduction of charges or punishment (though possibly substituting observation in a mental institution and/or mandatory counseling for jail) because it would tend to reduce or negate the criminal "intent" which is a necessary part of all crimes.