Your question is difficult to answer because there are several unknown variables. First, has your brother already been convicted of the felony? From the wording of your question, it appears that he hasn't. In that case, depending on the charge, it's possible for the crime to be reduced to a misdemeanor, either through negotiations with the DA's office or through the court. If he has been convicted already, then a 17b motion through the court can reduce the crime to a misdemeanor if the crime is a wobbler. However, if the crime is not a wobbler, which means a crime that can be charged as either a felony or a misdemeanor, then the felony cannot be reduced to a misdemeanor.
If your brother has been convicted already (ie. he plead guilty or was found guilty by a jury), but not sentenced yet on the felony, your brother's attorney certainly wants to bring this issue up in his sentencing motion and at the sentencing hearing so that the Judge can consider this mitigating factor.
The attorney should bring the issue up to the DA in settlement talks if your brother has not already plead guilty.
While, unfortunately and surprisingly to many, most mental illnesses don't qualify as a legal defense, they can certainly be considered as mitigating circumstances.