in order for her to change the name she would need you to countersign the V5 as you are the registered owner. If she changes it to her name, then yo know that she has forged a signature on the "registered keepr" section and then signed her name on new keeper or change of details.
Either way you must countersign her to be able to do this.
I would speak with the DVLA, each log book has its own bart code. You have 2 choices: -
1. tell dvla that you have lost the log book and ask them to send a duplicate, thus meaning the old one become null and void. You may have to pay a small charge, byt better that than the implication of what could happen.
2. advise the DVLA of te situation, ask them to either block the log book and send you a new one or make a note on their records showing that no name change should be made with out your prior authorisation, as this would be fradulence on your exes part.
As i am fleet manager it is always good to have a trick up your sleeve