I can understand why you fear talking to your girlfriend. You don't want to loose your temper and drive her into the arms of your friend. It is reasonable for everyone to be able to have friends both female and male. Your mind tells you that you don't want to tell her that she can't have him as a friend. Your heart tells you that you think it is a disaster waiting to happen. I believe you should go with your heart and express your concerns to her. Before you do, however, pick a person you respect and talk with him or them about your feelings. Pick someone objective that doesn't have a reason to side with either one of you. It will help to get an idea of just how to express yourself to her. It will help you get a little of the anger out of your system before you talk to her. It may help you reason with her better. The thing that makes this awkward is that her friend is a past lover and that is putting an undue strain on your relationship. If your relationship is more important to her, which it should be, then she should understand why you are concerned about her spending time with him. The key here, I think, is to approach the discussion with a cool head and expressing trust in her. If you make it clear that you love and trust her but feel it is asking for trouble from him, she will probably understand.There is no rule that says you must trust your friend with her.She can defend him all day long, but in the end, she cannot accuse you of not trusting her. I try not to say things that are sexist against men. Not all men are dishonorable. Your friend might be very honorable and never try anything because he values your friendship. My experience over the years has been the opposite. Numerous times, without cause I had close friends of my boyfriends or husbands try to betray their friendship by making passes. I did not encourage them or do anything to cause it. Rule of thumb, trust your best friend with your friendship, not with your girlfriend or your car.
