Sorry for the delayed response, but thank you all for your input. We had a very long talk about everything, and laid it all out on the table. I also showed this thread to her, and I have to say, although things aren't perfect, it's getting better.
I told her that the situation boils down to common sense and that if I don't trust him and if I don't feel comfortable with this entire situation, then she should respect it. Relationships aren't a one way street. Sometimes, she does have a tendency to play with fire, as she trusts very easily. I think the main reason why I'm so hellbent on this is because I don't want to see her hurt.