I think personally I would leave it alone like everyone else is saying, mainly because you work together. Most of my work life has been spent in offices and I've seen soooo much drama between people, I've learned to just be neutral and stay out of most of it. If you confront him about it, it will most likely blow up. Had it been a situation with someone you were friends with outside of work, it wouldn't matter as much if shit hit the fan because you would have more control in who you choose to associate with and talk to. At work it's a whole other story because you have to see these people all the time regardless of whether you want to or not. And honestly, if this Jack person is really as terrible as the other girl makes him out to be, his true colors will show eventually, no matter what.