I'm going to tell my friend to stop calling herself trans.
Why aren't you okay with her calling herself that?
Because I don't think she is trans.
You're entitled to your opinion.  However, it's not up to you what she identifies as.  That's her choice and her choice alone, and you need to accept that.
I want to make it my choice, hence I'm telling her to stop.
Someone's sexuality is nobody else's choice but their own. You shouldn't try to make that choice for someone else. You need to focus more on yourself and worry less about what other people are doing.