I.
Unjust vexation is punishable by imprisonment ranging from 1 to 30 days OR a fine ranging from 5 to 200 pesos OR both imprisonment and fine. Since it is only a light offense, the accused convicted of unjust vexation and penalized with imprisonment can simply apply for probation, and, he will not be imprisoned. Unjust vexation is a probationable offense.
If convicted of the crime of unjust vexation and the court sentenced him with a penalty of imprisonment, your brother can either (a) apply for a probation, or; (b) appeal the judgment of conviction.
If convicted of the crime of unjust vexation and the court sentenced him to a penalty of fine ONLY, your brother can: (a) simply pay the fine which is very very low or appeal the judgment of conviction.
II.
By plaintiff, I assume that you are referring to the complainant. If the complainant himself will not be able to testify, it will most likely weaken the case against your brother especially if the other witness will not be able to fully prove the elements of the crime through their testimony. If the prosecution cannot prove your guilt beyond reasonable doubt, then your brother will be acquitted.
III.
In mediation, the parties themselves must appear. If ever a settlement will be reached in mediation, it is in the interest of your brother that an Affidavit of Desistance be executed for the criminal case to be dismissed. The Affidavit of Desistance will be signed and executed by the complainant, not his brother.
If the complainant himself will not appear, then you can ask the Mediator to terminate the mediation proceedings for failure of the complainant to appear despite notice. The mediator will simply refer the case back to court.
IV.
The criminal case involves only a slight offense so the professional fees should only be minimal. Find a lawyer who will not charge much. The standard fees that you will incur are the (a) acceptance fee – fee for accepting your case; and (b) appearance fee – fee that you will pay whenever the lawyer attends hearing or whenever the presence of the lawyer is required.