Oh OP I’m so sorry you’re going through this. I think you are right that there isn’t anything you can do to stop her at the end of the day. I think right now, the most important thing is to try and keep open lines of communication with her. I would also emphasise how much I loved her and that there is nothing that would change that. If she does ultimately go down this path, the last thing you want is for her to do it without being able to turn to her mother if things go wrong or she has a mental health crisis.
I think if you tell her she is wrong and being influenced then you risk just triggering rebellion and even stronger determination. Instead why don’t you see if you could meet up with her and just ask about her life and how university is going?
You could then build up to asking her if she will talk to you about her gender problems. I would try to avoid taking an emotional stance although I understand how hard that is when you feel so distraught. Instead I would try to ask her open questions like:
Can you explain the way that you feel to me, and why you want to make this change?
How long you have felt this way?
What benefits do you think you will get from using hormones?
Have you thought about any alternative things to explore?
What do you know about the use of hormones and the risks?
Have you thought about ways this might impact on your romantic life?
Have you thought about how your fertility will be affected and your future plans in terms of having children?
This might identify some of the harmful ideas she has been exposed to. Again be careful not to attack, but you can gently question any harmful ideas. I would wait until after you have had an open discussion to do that though. Make a mental note and then keep a cordial discussion going, you can get back in touch with her and say something like:
Thank you for sharing your feeling with me, I’m really sad that you have been struggling with this and want you to be happy. There was something you said that I feel less sure about though. E.g. you said that you feel like a man because you like to wear men’s clothes. I know many women who also prefer to wear men’s clothes, and I think you should wear whatever you feel most comfortable in. Have you considered trying some things like this, and seeing if it improves your happiness, rather than going down a medical route?
I would also really recommend having a look at this AMA. It is from a young woman who has lived as a transman since being a teenager. She has some excellent advice for parents of gender questioning children, and ways that you can challenge harmful ideas without triggering defensiveness or closing down communication. I have based a lot of what I have said on that.
I would like to add a warning to you though that she has had a very difficult life including violence and mistreatment, so it may be too much for you to handle right now with your very legitimate fears.