I'm going to tell my ex's new boyfriend that she is manipulating him
How do you know she is manipulating him?
Her new BF is younger than she is. She is manipulating him just for this reason alone to start with.
If you know for sure he is being manipulated, then you should inform him. If you are doing it only out of jealousy, you should leave their relationship alone.
How do you know that I want to dictate the relationship between them?  I feel threatened by that.
Then just stay out of it and watch what happens. Things might just not be as bad as you think it is. If he is manipulating him and you have proof then it is okay to do something about it.