Attempting to fix this...
1) Hopefully your husband and his ex actually have a court order that reflects the current parenting plan. If the parenting agreement has changed, but it was not filed with the court, that would be the first step.
2) Did your husband petition to modify custody when the parenting plan changed? If not, then why not?
Again, you are a legal stranger. While I can understand your desire to help your husband, legally he's the one that has to do this. You can nag him, but ultimately he's the one that has to stand up and deal with this.