I'm going to ask my trans friend to stop copying my style
Why do you want them to stop?
He's not as good looking as me so he should stop trying to be the cuter one.
It can be weird to have people dress like you but they are allowed to and you cannot stop them. It will be rude and people will think you are a bully.
II don't care he might try to steal my girlfriend.
You are friends you should try to work this out. No person has ownership of a "style" they come and go in popularity. If you like something they are wearing you can ask where they got it.