I agree with arwen, not "allowing" her to go would have only made it worse. You cant make her decisions for her, and trying to will only make you both unhappy. You dont have to make her travel arrangments for her, or give the trip your blessing, but you certainly cant stop her!
My H took a trip to see the OW, he left his truck parked in the overnight parking lot at the airport and they called me to try to get me to move it. I told them to tow it... Then informed his command that he was on leave to pursue another woman, who was married, adultery is a pretty big deal in the military...hehehe. (unfortunately, they said that it wasnt necessary to tow it). But to this day he doesnt know that it was me. I honestly think that if he did, it would have destroyed any hope of reconciliation there may be in my sitch.
I guess I gave the wrong finger to the wrong man...