Keeping up with the calculators is very likely the hardest part of my job. I've had two go missing after nearly the whole year was gone. I'm very careful with them and actually wince when one is dropped in class. I emphasize the importance of taking care of them to the kids on a daily basis.
Some of my students have their own and bring them to class to use. Occassionally, one of those kids will accidentally take one with them and later bring it back red-faced and apologetic. I am understanding about the whole situation.
It irks me to no end that I don't know what happened to the two I've lost. I know other teachers have lost a few over the year, and that brings me a little comfort, but not much.
Something interesting.... While we were conducting our state testing, I found a calculator on a student that belonged to the school and had been missing for over a year! I confiscated it and returned it to my department head. I believe they are pressing charges on the girl.
I'm sorry you have to deal with such a difficult person. I hope your relationship will improve.