Sucks for the kid. Maybe eventually mom and dad and new woman can all be adults and sit down to work things out.
Sounds to me like it's best left between the adults and their lawyers and the judge. Extra people outside those three will just draw factions and inflame everything.
A lot of the evils end up happening when outside influences start egging on the participants. "Yeah, you are right and you should totally tell him to screw off because you're the good one!" would just make it less likely that anyone will want to work things out.