I think I'm just going to threaten to tell on my best friend's new girlfriend. What do you think?
How will that affect your friend over the next year?
Most probably he will break up with her
If you really think that your friend's girlfriend did something that is concerning, you should talk to your friend about her behavior as friends watch out for the well being of each other. It's not good to only threaten to tell when it's something that your friend deserves to know. Your friend should be able to make an informed decision about his relationship and he isn't able to do so if you withhold information from him.
Nah, I'm still going to tell on her. My friend won't listen to me if I just tell him about it. Part of friendship is looking out for your friend and a best friend would want to know.
I still think talking to your friend first is the best way to go. If that doesn't work you can always go ahead with your plan. You may ruin your friendship by not giving him a heads up about the situation.