I feel the need to let this trans girl know that she will always biologically be male
why would you tell her that?
Because it's true
You shouldn't force your beliefs onto others. If they identify with something then respect them and move on.
Eh, I don't agree with that.
Think of how you would feel if she tried to force you into changing your gender. That's how you'll make her feel by dismissing her belief in her identity. It would be nice if you could just leave people alone instead.