I think your feelings are totally understandable given the very sticky situation. I'm not clear why your T had to cancel your session to give it to your friend, as opposed to canceling the friend's session if the time conflicted with an event she needed to attend--but the echoes of your past experiences is painful, especially coinciding with your b-day.

The only thing I can think is that if your friend isn't so happy with your shared T, and your T agrees that it was a mistake to have taken on your friend as a client, perhaps the relationship between them will be short-lived regardless of anything you do or don't do. Really all you can do is spell out your feelings to your T and get her cooperation in separating the relationships completely. And if your friend doesn't abide by your boundary--somehow I feel like she is going to slip--then you may have to cut her off for awhile.