What everyone else has said... that friend isn't a friend.

You don't need to keep anything secret from your T - in fact, you should be trusting your T and talking to them about what is bothering you. And understandably, it is bothering you that your friend isn't respecting your boundaries and is in fact using therapy as her excuse. Like... if your friend didn't keep going against your wishes, you wouldn't hear things that upset you about your T. That would solve the feeling of guilt for hearing things about your T and the pressure you feel that you have to keep a secret. You don't need to keep any secrets from your T.

And to then hear that your friend got your regular appointment time? I would definitely bring that up to your T - it's totally reasonable for you to be hurt by that. Especially on your birthday week - do you find your birthday triggering? I sure do.