Sage, first of all..wear a gown and be gorgeous at the ball! Have your hair done, if you can, and nails, and be ultra feminine and ultra enchanting. Make him proud to show you off at the ball! And if he doesn't act proud, then show yourself off by being gracious, gorgeous, and secure with his friends...both male and female.
Now, suggestion as to his friends...try to practice some acceptance. He is in school. It is coed. He has friends of both sexes....accept that. Don't overlook danger signs, but don't read too much into things either. He may be mentioning ff a lot thinking if he is talking about her you won't be suspicious or inclined to believe there is an A going on with her. He may be trying in his own way to calm your fears..not create more. Would he really be talking about her if there was an A going on, after all you have been through?
Focus on the good, as you so well know how to do. You are an awesome woman and his ff probably can't hold a candle to you...after all he married you, didn't he? Be the girl of his dreams at that ball...laugh, smile, flirt a bit, but also be attentive to him...tell him how great he looks and how good it feels to be on his arm...to be his lady love... If you become Cinderella, then he'll become Prince Charming by default! LOL