Well, it's his wedding, too. Presumably, he wants to invite her because he would like her to be there to share his moment.
It's quite mean to veto his friends, but if it is going to really cause you grief, then maybe you could just invite her to the reception? (If it is the thought of her witnessing your vows that is causing you problems).
I had a similar situation with an ex of mine, whose GF went ballistic at the thought of me being there. It made me quite sad, because he was a really good friend and I was very happy for him, and would have loved to see him get married, but bridezilla won out. Plus she gave me much more importance in his life than I warranted, but hey ho, her decision.