Please help

Dear wellwishers, I am going through a very ard times. Me and my girl friend have been going steady for 8 years. Recently I had a very serious doubt about her with a very senior person ( a grandfather actually). I tried to evesdropping, Audio/Video recording and caught them having sex togather. After confronting my GF and that person with the evidence and after a lot of heated arguments and heart aches, as I cannot be without her, I forgave her for all her misdeeds and finally she also promised me never to do it again and never speak to that person ever again. She follows a religion where they have to carry out voluntary works for the needy. The problems is she insists and fights on going with that person inspite of me constantly refusing, the reason she gives is as she has been going with him since so many years she cant all of a sudden refuse to go with him. I am in a fix as I did some more eavesdroping on them and came to know that he has been trying to wind her up in terms of forcing me to talk to him and go with him to the voluntary works for their religion. The travel up north for about an hour and a half every weekend. I get very ancious as lot of things did happen during their travel in his car. This happens every weekend and we end up in fights near and after every weekend. I do love her a lot and dont want to lose her. Please help me I feel lost as with constant fights our relation is going down the drains and I cant tolerate to be with that person anymore. I also know that person and his family very well and this is stopping me from spilling the beans to his wife as they have not done anything wrong and I feel its wrong to snatch away their trust in him. Please help me guys. Please. I am totally lost.......

If your wife insists on going then you need to take her (in your own vehicle). She should never be alone with this person. Is there a reason you can't provide the transportation?