Yes, I think it's fair. She should have known better. My T (and several others) has a strict no relatives/friends policy no matter what the client says.
I don't think you're overreacting. Honestly, if my T gave my slot to s.o. else I'd be pissed and would need to talk it over and unless there were some really good reasons behind it I'd terminate. Giving it to my friend who I knew is not in some imminent danger? WTH
I think you still have an option here- you could ask your friend to give up her T. I do remember how hard it was for you to find this T and it'd possibly be that hard for your friend too but still. I'm not saying she should do it but she may and at least you'd make her aware just how hard it is for you to have to deal with sharing your T and it can make her stick to the boundaries you established (good job, BTW)
What I think you should do though, is having a talk about this all (how bad it's making you feel, ask her not to discuss your friend with you, even if its a compliment, and most importantly the session) with your T. Saying you wish you didn't do it and her admitting a mistake is just not enough.