It's SO frustrating...
I was and kind of still am in the same boat as you. The mother of my fiancee's son is the CP, and she thinks she has all the control. She wouldn't even give my finacee a medical card for their son - she told him that if he was sick then the doctor could call her and she would give the doctor the information. Like that? We got the card by the way, by going online and having it mailed to us.
What I am finding out as time passes, control freaks will eventually screw up. She has been caught lying a couple times since the custody/support agreement went through, and to the wrong people. We don't NEED to take her back to court YET, we'll just let her continue to screw up until she does a big one.
If your fiancee doesn't have a court order in place, he needs to get one. I know it gets expensive, but it is SO worth it. She will still try and pull stuff, but she only ends up making herslef look like an ass. MySonsMom is right - joint legal custody means they BOTH make the decisions. Whether she likes it or not.