Show her the average mens toilet and ask her if she really wants to sit down on that.
On a more serious not this is very sad. It’s hard for us to know how involved in her life you are, as uncles vary a lot. I don’t know what you can do other than being a voice against it and hope a part of her wants to leave. However, being a naysayer will eventually get you kicked out of any tranny’s life. You need to be as loving and kind as possible to delay this. IMO