I do agree with the others, no more on the cell bill, AND any other bills that should be her's.
You need to document what all happened as far as this incident goes. Write it down while everything is fresh.
You also need to flat out be strong with her on this. "W you taking our D over there is totally unacceptable." "I will not put up with it." You MUST stick to whatever boundary you lay down.
Use that anger you have as a shield, don't go on the offense with it, but do use it to toughen up, harden your heart a bit, and tell her flat out that you will not tolerate this. There needs to be consequences to follow. Start planning what you will do IF she crosses your boundary.
There is a balance to this, some of the little things you let slide and live to fight another day, however this isn't one of those. You need to stand firm on this.
Don't lose your cool, however be very very firm and as a matter of fact.
