That's a sad situation but I seriously doubt anything can be done now. The state is the one that charges and prosecutes people for crimes, not citizens like you. This is a common thing with dv, you would be surprised how often this happens. It happened daily when I was interning at 2 prosecutors offices.
If he has been convicted, there is unfortunately not much that can be done. If he plead guilty or was found guilty, the conviction is pretty permanent. Depending on the charge, there might be a chance to have the record vacated down the road but it depends on what the conviction was for. For example, if it was a violent offense as defined under RCW 9.94A.640, then the record of conviction may not be cleared. It is best for him to consult an attorney to vacate a a conviction and usually a time frame of 5 or 10 years or more has to have passed.