A hypothetical scenario, if one of my friends pleaded guilty or nolo contendre for a misdemeanor such as PC 415-disturbing the peace, would he be able to lower her misdemeanor into an infraction after finishing her probation time?
In either scenario, assuming probation is completed without any violations/new offenses, you can petition the court to have the conviction expunged and the court WILL (has to) grant the request. Just don't mess up and you're golden. Don't worry about the infraction bit. Just get the expungement.
Typically, on a charge like PC 415, a reduction to an infraction is negotiated with the prosecutor before a plea is entered. If your friend entered a plea to a misdemeanor and was given probation, he or she can attempt to terminate her probation period early based on compliance. Once off probation, that person can file for an expungement, which is a procedure that allows the charge to be dismissed. Obviously, getting the charge dismissed is more advantageous to your friend than reducing to an infraction at that point.